For best performance you should have the mainsail leading edge as long as is allowable, but not so long that you cannot tighten the luff in heavier air and loosen it in the lighter stuff. In other words, if you need to tighten the luff of the sail so that it jams into the top of the mast, then the luff is too long. You say 10". I assume the luff has little or no tension at that point. It is just up but no more. If the winds pipe up bigtime so that you want to raise the halyard to tighten the luff (which brings the draft of the sail forward), how much of that 10" is left with a tight luff? Might be ok depending on the stretch of the sail. Obviously if you get a tension curl at the leading edge (luff), then you are too tight.
