answered in other thread you asked, but will answer here too for benefit of those not in both games:
In most cases I think it'll be fairly easy - If you fail a save by enough that you'd be knocked out, I use a luck point for you. If you have any other circumstances, just post them with your post (IE "Power attack 5, willing to use Luck/HP to ensure hit"), and if something comes up that we haven't forseen, Just say "Could I get a reroll" and I'll do it asap.