Great discussion ! To answer your last question, 3 in of sag is not much at all, that's pretty tight. However, your tacking angles in 18 kts should be way less than 90, more like 80. You've basically got the concepts right, tighter rig especially lower makes tighter headstay. Here's my quick, easy tuning guide, when sailing upwind your caps (uppers) on leeward side should be just barely loosening (flat water tighter, choppy looser ). The lowers should be tight enough to keep the mast in column side to side ( light air , a bit of leeward sag side to side ). This will put you in the ballpark but you've got test and retest and record your own settings and numbers when you are fast.
The other trick is don't use the backstay unless you really have to. Keep tightening the rig, tightening the mainsheet and tighening the vang, especially mainsheet.
One other thing to mention is that your prebend needs to match the luffcurve of your main. The prebend numbers you guys have been discussing are pretty low and may not match how your main was cut. Good luck and keep up the great discussion.
