The weight of the broadhead should be decided after you've choosen your bow and the arrow you intend to shoot.IMO There is a great article on this home page which talks about Front Of Center.FOC is expressed in a percent and basically just tells you where the balance point of the arrow is. FOC will determine how your arrow flies after being shot. FOC between 7% and 10% will provide you with the flattest possiable trajectory and FOC generally range between 7% and 18%.
Here is the link to the article. I'm sure it willl answer a lot of question you may have. I know it did for me.
http://www.bowhunting.com/everythingarrows.asp?page=5

I hope this helps some