I'm not sure where you got that figure, but I'm am one who does not agree with it. When talking about FOC with fixed blade broadheads, my opinion is that the higher, the better. With my compound, I'm shooting approx. 18% and I'm up around 25% on my longbow. The only possible negative is that trajectory will drop off faster. Within typical hunting ranges, I haven't found it noticeable. A 500 grain arrow with 18% FOC seems to fly about identical to a 500 grain arrow around 10%. The differences probably show up more at longer ranges, which is why hunters typically do not have to be concerned with it.

There are a few significant advantages of very high FOCs, so I'd recommed not decreasing yours.