Well as long as you don't wait too long it isn't really going to matter whether or not you LB it at 100% or 50%, the LB doesn't lower their resistances to anything so it's essentially the same as casting a stronger Heavy Shot at it. You're not really "wasting" damage unless you wait until the LB would do more damage than health it currently has.
I usually wait till the boss is at 15% before I use my LB, but this is mainly to make sure that the Melee DPS (I main Bard) isn't going to use theirs, and at that point I might as well go for it.
