San Juan 21
First, it is important to have the headstay length correct. We use 24'4" pin-on-mast to pin-on-deck with good success. This gives a fair amount of rake, which improves pointing. This usually requires a 3-4" link plate at the bottom of the stock headstay.
I have our uppers set at 21 on the Loos gauge and the lowers set at 29. With this rake setting, you may have to re-work your backstay adjuster to get enough play to allow the rig to go all the way forward for downwind and still get enough headstay tension for a big breeze upwind. The headstay will be quite sloppy when the boat is sitting in the relaxed mode - makes standing up really easy!
We sheet the jin with car approximately 9'5" from the stem fitting. This is the "normal mode." Wind range from ~5 to 12-15 true. Move the car back slightly for less than 5 knots - this allows the upper leech to stay open while you can sheet the foot tight enough to point. Also move the car back when it's really honking - this relieves some of the heeling pressure and lets you feather up in the puffs without losing the whole sail at the same time.
Normal range, clew about 4" from teh car. The leech will touch the lower shroud and maybe turn over a little but that's no big deal. If the leech flutters, use the leechline just enough to stop it and no more. Once again, you'll see a little leech curl, but that impact is minimal in the big picture. Use only enough halyard tension to just smooth out the wrinkles at the hanks, and sheet hard enough to fold up the lower section of foot shelf; the upper section should still have some curvature. Keep an eye on the mid leech telltale and try to keep that flowing back all the time.
The main will definitely hit the backstay. In light air, if it hangs up, dump a foot or so mainsheet to free it and re-sheet. This is actually fast, as it insures that the main isn't stalled when accelerating out of the tack. Sailing upwind, in normal more, keep the three lower leech telltales flowing all the time. The top can be stalled 50% of the time as long as the top batten stays parallel to the boom or more open. Use enough backstay to just lose the backwind at the front of the sail - the cloth should still look a little soft, but the leeward side mid body tell tales should be flowing and you'll probably be able to get the weather side ones working too. Keep the leech open and go fast before trying to point.