I don't believe that it's super important that you figure your FOC accurately, just that you keep it high. Keep in mind there is no upper end to what you can shoot. Don't think 10-15%, think 18-20% or even higher.

The FOC becomes important when conditions aren't the best. Arrow stability is needed in wind or when there is a chance to hit grass or small twigs. Some lower FOC setups will shoot okay in ideal conditions, but are far less stable in a stiff breeze. There have been some recent studies by Dr Ashby on extreme FOCs in the 20-30% range. He has found that penetration is substantially greater in these upper ranges. I've been hunting with arrows in the 20% range with my compound and in the 25% range with my longbow. I must say, I will never hunt with lower FOC ranges again. The accuracy of these extreme FOC arrows is simply better in tough conditons than any other I've ever tried.

I've tried to make extreme FOC arrows and it's not easy getting the tip weight as high as I'd like. Take a good heavy hunting arrow and put a 100 gr insert and a 125 grain broadhead and you still will have a tough time getting to 20%. You have to search the traditional hunter's market for heavy broadheads. If you put a lot of weight up front, chances are your arrows will fly great, if you build your arrows to handle that weight. If you try to shoot light weight arrows because someone has convince you that speed is important, then you can expect less consistant results in wind or thicker cover.

When experimenting with FOC, you do need to make sure your arrow spine can handle it. When adding a lot of weight to the tip (like a 100 grain insert), you will probably have to switch arrows, or cut the length down. You could also reduce draw weight to get your dynamic spine proper for your setup. Going from a 100 grain tip to a 125 doesn't affect much. It also doesn't change FOC all that much, unless your shooting a very light arrow.

In my opinion, every hunter owes it to themselves to take some game with extreme FOC arrows so they can see the advantages for themselves.