I have set up several crossbows and have used some of the setups on deer out to 30 to 40 yards and what we have found is that a 125 grain head seems to work the best. A head that is 125 grains and is relatively small like the rocket ultimate steel work great The heavier head helps with penetration, where as a lighter head of open on impact design did not penetrate nearly as well. Also there is a FOC calculator on the web, here is the website:
http://home.att.net/%7Esajackson/archery9.html
Other than that I have run into very few setups where fieldpoints shot exactly the same as broadheads especially with fixed blade heads, open on impact heads seem to shoot closer to field points but did not work as well on deer using the crossbow in my expierience