Too much FOC hurts your trajectory,
Although I'm sure this statement is true, I've never found it to mean much at hunting distances. I've shot 15% and compared it to 12% and couldn't detect any significant difference inside of 30 yards. It can be difficult to find a broadhead/arrow combination that will weigh the same, yet give large differences in F.O.C. It's a natural result of increasing FOC, to have a greater arrow weight. This as much as anything is what hurts trajectory inside of 30 yards, and even that isn't much concern unless the arrow is real heavy.
9% would work if you had good spine and a well tuned bow.
I would probably add - with no wind and no twigs or grass possibly in the way.
Although I don't find a particular FOC that is best, I do find that the higher the number the better arrow flight is. Few people have to worry about getting too high. It's difficult to walk into many bow shops these days and even find a broadhead that weighs over 160 grains. I did a little informal survey of the guys in my winter league and asked what grain broadhead they used. I only asked about 25 guys, and out of that number the highest was 125 grains. You would have to be shooting a very low poundage bow with a very light arrow for a 125 gr head to get the FOC over 12 or 13%.