Set your bolts (and, yes, the short "arrows" used for crossbows are called bolts) up the way you want them and don't worry, too much, about FOC %. If you are getting good flight, just shoot them.
If you aren't getting the results you expect, or if you just want to experiment, it's simple to add weight to the nock or point end and check the results.
The ideal FOC % is purely subjective, depending on your equipment and what you are aiming for. Some short yardage and 3-d shooters try for 7-9% while some longer range shooters try for 12+%. The fact is, the majority of bowhunters out there will give you that "deer in the headlights" look if you ask them what their FOC % is.