How you trim your backstay depends on your tuning and the cut of your mainsail. For the usually light air Chesapeake, we setup with very little mast prebend and loose backstay. The mainsail is very full with a loose luff and reasonably eased halyard, for a few 'speed wrinkles'. We might pull on a bit of backstay if the mast is bouncing in the chop. At about 8-10 knots of breeze, we snug the halyard, and start to tighten the backstay to bend the mast and stretch out the luff in. If we anticipate 15+ of breeze, we set up with tighter shrouds, more prebend, tight luff/halyard and a snug backstay. Then the backstay wouldn't need to be trimmed until about 12-14. I've sailed on other J/30s that use less backstay. We generally have good speed can't point as well without using more backstay.
