Isn't this less about overlap and more about room to maneuver? Likely B will have more speed than A who is coming out of a tack and so the distance is measured in ability to avoid rather than a number of boat lengths? 

I tend to agree more with 5, as you cannot measure the "correct" position with a ruler. This is about giving space & time to the ROW boat, and the amount of space and time depends on a number of factors, like boat LOA, current speed, sea state, wind speed, and the general ability of the boats to manouver (not the same to move a IOM 1-metre boat that can turn on a hairpin than to move a 50-foot maxi).

This is an old conundrum, which I always face by asking parties, in protest hearing, how they _felt_ about the manouver. Were you afraid at some point? Did any crew fear for his/her life?   And the obvious questions for the ROW boat: Did you take evasive actions? If you would have acted differently, what would have happened? 

Difficult call always, as as I said, depends a lot on the boats and conditions. What the decision should _not_ be about is about the crew experience: pros and beginners face the same rules and same conditions, their expertise is not no favor one decision over the other.

BTW: this really becomes a challenge if incident involves different boats in a mixed regatta.... think a fast catamaran vs a slow monohull... probably both skippers will misjudge other's behaviour n the water.

There is not a distance in meters or boatlenght. First opinion is not supported by any rule, it is just a mito. Second opinion.... Well, in the end, the only requirement on the tracking boat A is to switch off rule 13 before the STB boat B has to manouver to avoid her. After this, rule 11 on A and 15 on B applies and both have to avoid contact keeping clear and giving room. If A become overlapped to windward of B, B has not to give her room for keeping clear. The class, size, fleet and model of boats doesn't matter.

Sticking to the rules:
PT must keep clear of ST until PT is on a close-hauled, starboard tack course. (Rules 10 and 13).
When PT assumes her starboard tack course, Rule 15 applies and ST must keep clear (Rules 12 and possibly 11).  ST need not take avoiding action until it is clear that the other boat is not keeping clear or giving room.

Whether or not PT breaks Rule10, 13 or15 depends a lot on the types of boats and conditions and cannot be assigned a fixed distance (2 boat lengths in your example).  It has more to do with the time that PT gives ST to respond.  (Two J/70s at an upwind speed closing speed of about 3 kts = 1.52 m/s = ~ 5 sec/boat length.  10 seconds is a very long time.)

In general, though - if ST did not take avoiding action until after PT completed her tack and there was no contact, then both have complied with the rules and there is no foul.

I agree with Matt's closing statement, but would add that ST should not be obliged to take avoiding action that is un-seamanlike, and especially does not cause or be likely to cause damage or injury.
It is relatively easy to judge if room to keep clear has been given if you are sitting in an umpire boat. I strongly believe that umpires are experienced enough sailors to recognise one of those moment's when you hold your breath and wait for the collision... It is more difficult in the protest room. 

While Luigi is right that the rules provide no fixed distance, or reference to any specific classes or sizes of boats, I must respectfully disagree that those things don’t matter. Instead, I second Matt’s view that the answer will be highly dependent on conditions and boat type. For example, on a beat in a highly maneuverable boat like a J70, if P completes her tack 2 BL ahead of S, that’s tons of time, almost certainly much more than S needs to keep clear. On an off wind leg, particularly in fresh breeze, the boats will likely be moving much faster and more time may be required. A race of IODs, super yachts, catamarans, or foiling Moths will each produce different times, speeds and distances, as will different points of sail.

I also agree with Gordon that P may not force S into an unseamanlike maneuver to keep clear, but in such a maneuverable boat as a J70, a fairly abrupt change of course will likely still be perfectly seamanlike in most circumstances. In IODs or classic yachts, not so much...

This is also a good place to have a look at Case 50. It says that PC decisions about protests over situations like this should hinge on whether the ROW boat has “a genuine and reasonable apprehension of collision” and acts to avoid it. As Gordon notes, this is easier to be sure of in an umpire boat than in a protest room, but it’s not impossible in the latter either (that’s why they pay us the big money! 😁)

I am in agreement with Matt and Gordon.  Once PT is on a close hauled course 13 is “off” and 11, 12, and need to be examined. I tend to ask both skippers for speed and time to get a sense of the physics involved and if there is agreement on those things, treat them as facts to draw conclusions from.   In these cases an outside witness can be valuable!  But from the read of #3 no avoiding action was taken by St and no contact occurred therefore there may have been a little “pucker factor” on the part of St, but I don’t see any rules that may have been broken. 

I am always amazed when a competitor believes they have a claim to room and that right to room was somehow violated, when no rule supports the claim. However, I think is is a good candidate to be resolved by arbitration. 

So the answer, as so often, is 'It depends...'

Gentlmen, please stay with the Rules. Is it written anywhere in RRS anything about applying RRS differently depending on boats or weather conditions? Many of the Rules will be broken or not depending on the outcome of the manouver. 

No, the rule doesn’t provide different rules for different classes, conditions, or points of sail. Instead, it leaves it to the sailors (and to PCs) what is a reasonable and seamanlike effort to meet one’s obligations. I respectfully submit, though, that what is reasonable or seamanlike in one class, point of sail, or set of conditions may not be reasonable or seamanlike in another. An Optimist dinghy in six knots of wind and flat water has different limits than a J-boat in twenty-five knots and a sloppy sea, no?

It is certainly true that the outcome of the maneuvers will bear on whether or not they are seen as having been reasonable or seamanlike.

The rules do speak of "seamen like" efforts - and those depend on many factors, which are I think omitted from the rules on purpose, _because_ they depend of the class, venue and climate.

Given this omission, I believe the key way to approach this is by talking about space & time given to the ROW boat. This approach has always helped me move away opinions and to clarify facts.

I think we are staying within the rules when it comes to applying the definitions of "room" and the interpretation of "seamanlike."  Both depend on the type of boat, the sea state and the wind velocity.

Definitions:
Room   The space a boat needs in the existing conditions, including space to comply with her obligations under the rules of Part 2 and rule 31, while manoeuvring promptly in a seamanlike way.

CASE 103
The phrase ‘seamanlike way’ in the definition Room refers to boat-handling that can reasonably be expected from a competent, but not expert, crew of the appropriate number for the boat.

Seems to me this is the crux of the matter. If B didn't change course and contact didn't occur, A kept clear.

Other examples (not specific to this scenario):

WS Case 21: "When a right-of-way boat is obligated to give mark-room to a boat overlapped inside of her, there is no maximum or minimum amount of space that she must give. The amount of space that she must give depends significantly on the existing conditions including wind and sea conditions, the speed of the inside boat, the sails she has set and her design characteristics."

WS Case 134: A boat's proper course at any moment depends on the existing conditions. Some of those conditions are the wind strength and direction, the pattern of gusts and lulls in the wind, the waves, the current, and the physical characteristics of the boat's hull and equipment, including the sails she is using."

to me, this is a tempest in a glass of water. It's all in the book.

Port tack boat has to "keep clear". First for RRS 10, then for RRS 13 if taking, and if she ends up overlapped WW, for RRS 11, or, if overlapped downwind, "room"  for RRS 15.

"Keep clear" and "room"   are rather well described in the "Definitions", And applies to all boats,  in all conditions. 
Of course different boats and different conditions  will makes different speed, distance, timing  etc, to match the requirements of  "Keep clear" and "room" , but in principle,  the Keep clear and room are, well, rather "clear"