It depends on your arrow spine, draw weight, arrow length etc. I use the various arrow charts to see what arrows I should be shooting, and then pick the broadhead weights that fit. Most use heads in the 100 to 125 grn range. I suggest visiting my website
www.broadheadtests.com and read over the various tuning sections along with the mech vs fixed page. That should help you better understand the tunning process as well as help you pick the right head for your setup.