Chris is right on in his reccommended percentages for target shooting.( in my opinion anyway.) For hunting, I like 12-14 percent F.O.C.
What your F.O.C. does, in a basic way, is guide your arrow. The weight of the arrow in front of center is what guides your arrow towards the intended target, hopefully in a arc, with the arrow point leading the way. When a arrow is off in F.O.C., one type of 'wrong' flight is the arrow skimming towards the target without the arrow point leading the charge. Kind of like when you skip a stone across water. The front of the stone is up in the air a bit with the belly of the stone hitting the water for the skip.
Hope this helps.
Tweak, tweak, tweak...