there are 3 points to consider

1) I agree with dick that FOC comes into play, though I think 7 - 10 percent is a little narrow. IMO you can expect good FOC up to around 15%.

2) SPINE. You need to know where your setup and arrow spine sit. If you go to heavy or to light it can adversly effect spine and flight.

3) SPEED. Once you find the weight range your setup will work with then its up to you, if you want to milk a little more speed go on the light end.

I do agree to an extent with justhrowit, in that shooting a few heads is a good way to find out, BUT.....not everyone has access to dif heads or the money to buy a bunch of packs. So, if you do a few calculations that will get you a reasonable idea of what you need is much better than just winging it and buying 100 dollars of broadheads you may never use.