I've seen numbers as high 15% suggested. And not every one measures FOC the same either, there seems to be a dissagreement on which is correct. My feeling is that 9% would be a bit on the low end for a hunting set up. Not that it couldn't be done though. And we need to keep in mind that Bigpapa shoots pretty light arrows as well, not to mention his bow is most likely tuned very well.
I don't beleive there is an optimum value. Like everything else in archery it really depends on your set up. The type of head you shoot, length of your arrow, weight of your arrow, spine of your arrow, type of fletching you shoot and how well your bow is tuned will all play a part in your FOC.
Some would have trouble getting as low as 9% FOC with a conventional set up, for others it would be the other way around.
Basically what FOC does is cause the nose of your arrow to want to go down in flight. This would in turn cause your tail to go up forcing your flethchings to control the arrow more. Too much FOC hurts your trajectory, too little could make your arrow unstable and unforgiving. A large arrowdynamically challenged head may require more FOC and better fletching to fly well.
The point is to not go real light on foc if you are going to shoot a fixed blade head. 9% would work if you had good spine and a well tuned bow. I have done it with less. I have also shot with 16% and had no problems either.
And the point some of the others are trying to make is that it is about the last thing most of us consider. More important would be spine in my opinion. And if you changed to a heavier tip or lighter fletch to get more foc and your arrow flew better, who is to say it was the FOC, or the fact that you may have changed your spine for the better?
Most common arrow set ups will yeild plenty of FOC, so I wouldn't fret over it. And you can go to Bowjacksons site and it will figure your foc with different components to see what you have or what you could get.
http://home.att.net/~sajackson/archery.html
At least that is my opinion on FOC. Nobody is really right or wrong on this issue, it's just what works best for them in most cases.
Paul