I noticed that your topic has to do with broadhead weight and not type. I agree that the FOC is an important consideration in determining this. I have found from my own experience that I prefer an FOC in the area of 13-15% for fixed bladed heads. You may be able to go down a little more to maybe 10% for mechanicals, but I don' t use them, and my experience is limited. Currently, I am shooting just over 14%. I use a few different broadheads (all fixed). My main hunting head is a magnus four blade (185 grain with insert), but I am also messing around with a 125 grain GFK Deadhead. For this head, I use an larger insert with a 50 grain weight to get the FOC to were I like it.

Here is a good place to start to check for FOC without doing your own calculations, all you need to do is click on the type of arrows that you use and fill in the rest of the info:

http://home.att.net/~sajackson/arrowtools.html