Being the starboard boat, I would have come up to my best VMG heading. 3-4 seconds is enough for port boat to take evasive action. Unless you were on a 60-footer.
Port must give way to starboard, starting procedure makes no difference. 

Remember that the term "proper course" has no meaning prior to the start.  As long as you gave room for them to keep clear when you made your course change, you complied with 16.1. They always have the option to tack or duck.

G Holt:

There's nothing that gives you the right to assume your "proper course".

Obligations:

- Port boat is obligated to keep clear (rule 10)
- Starboard boat (you) is obligated to give room to keep clear when changing course (16)

If you had come up to the course you wanted, would the port boat still have had room to keep clear? (That's the key question - sounds like no, but hard to tell for sure.)

Sounds to me like everyone did what they're supposed to. If you had headed up, and port boat couldn't keep clear afterwards, then the penalty would be on you.

Mr. Holt:
The question is if you had altered your course at the starting signal by heading up to close-hauled, would the port tack boat have had room to keep clear. As Mr. Armstrong points out, "3-4 seconds" is a fairly long time. In a boat of, say 6 metres, which can usually turn fairly quickly. At six knots, four seconds is two boat lengths, which is probably enough room. If, OTOH, the distance was a boat length or less, it may well have been too little space, depending upon a variety of factors.

I agree with David's assessment on the rules.  It may also have been in your best tactical interest as well.  Unless there was enough room to force a duck you probably don't want to make a change that puts you into a lee bow situation right off the start.  The pin end was obviously favored but if they were a full length in front of you keeping clear and they were not over early you may have had more room to head up before the start.

It sounds like you played it just about perfect.

My penny worth is exactly as David said it. If S changed course and there was a collision, the situation becomes a bit subjective and maybe depends who has a witness support.  Here's a drawing of how I see the scenario :-

In my current "sport" of RC racing, 4 seconds would probably have been enough! We'd expect 1 second from when the new course was a collision course for the KC to begin to react, and if he tacked, using up another 1 second, then that gives 1 second for the starboard (now astern and KC - R12) to react so that might just be enough.  But on bigger boats, even 5 m dinghies, you'd need more time.  In the above scenario, P1, P2, P3 boats are not on collision course. Only at P4 might there be a collision course, and only then might Yellow need to begin to keep clear - but he could argue that P4 was not collision and only when Blue changed course further to P5 was the collision inevitable (Yellow does not have to anticipate Blue's course change) and by then he had no room to Keep Clear. If Yellow at P4, identifies a likely collision, he'd be at P5 before he could begin to keep clear - and that would be the collision. In RC racing in IOM (International One Metre) we cover 1 boat length per second. A 5 metre dinghy at 4 knots, covers one boat length in about 2 seconds. My subjective view is that 4 seconds from beginning to alter course(P3), and probably 2 seconds from possible collision (P4), in a small dinghy, that is insufficient for the Port tack yacht to Keep Clear.  So, if Starboard had begun to change course 4 seconds before start, she would have been 2 seconds from collision due to her still changing course, which is totally insufficient room for Yellow to Keep Clear so Blue should be disqualified R16.1.

wow. I came to the right place. These are awesome conceptualizations, points, and considerations.

- yes, we were both at, and were targeting, the pin end. And yes... regarding my timing to the line, I probably could have come up earlier, and thereby could have established my course earlier with more clearly enough time for Port tack boat to adjust.

- upon further review of video I have (yes. I have video! had a camera on my port quarter. I probably should have just posted it for review?)..... it appears as though the time between the starting gun (the point I intended on changing course to windward) and potential collision time was more like 2-3 seconds.  And we were both in 14 footers, each doing perhaps ~ 6-7 knots.  Would that have allowed port tack boat room/time to avoid? I don't think time to duck...but maybe time to tack? Still not sure.  Only thing I"m sure of is that this was a friendly club race on a mountain lake in February.... so none of us had any interest in swimming, or causing someone esle to swim :)

- yes, tactically speaking, I get (now) that forcing them to tack into my lee bow position probably wouldn't have been advantageous for me anyway (of course,  not so smartly, I ended up tacking right onto their windward hip after clearing)

- in reality (and I didn't mention this in original set up), I actually did make a quick and minimal last-moment duck to be sure I cleared their stern. BUT...since I 'may' have tweaked up a little after the starting gun (and potentially too close to give them proper room according to 16.1)... then said duck seems like should indeed have been my responsibility.  (I guess it all hinges on an assesment of "room to keep clear")

- and yes, in the end, and given the spirit of fun and safety, I agree both boats handled the incident perhaps as best as we could/should have - maybe even in tactial and rules terms too. But for the sake of developing a better understanding of the rules...I definitely appreciate all the comments and expertise.

So thanks again, all. And continued safe/fun sailing!!!!

(and Mr. Campbell - based on my original explanation, yeah, I'd agree your illustration is spot on. In reality we were both closer to the pin though, and he (port tack boat), was actually closing in on the pin from a more windward position before edging under it and pointing up ahead of me, and my "pre-start" course had me just clearing the pin. I"ll have to look into making those illustrations myself!!! : )

If they were crossing, and then you headed up, and then back down (to give them room to keep clear) then it sounds like your situation is pretty similar to one we had in the news recently: https://www.youtube.com/watch?v=go4HOg7O_uI

G Holt, and anyone else interested, the drawing program I use is "Boats Scenario".  I think this link will get you started. http://boats.sourceforge.net/

Boats Scenario latest version is August 2020 
It has different hulls available for use in drawing and some other new features
Good time to update if you are on an old version
 http://boats.sourceforge.net/ 

Thanks Stewart