Carbons are a different animal and all are not created equal. Stus calculator may get ya close, but experiment you will be surprised. I know guys shooting 425 grains up front out of 45-50# bows using .500 spined arrows and some of them full length and the guys draw 28"s and have great flight. I was playing today and was shooting .600 spine 30"s long with 190 grains up front and getting bullet holes thru paper and perfect flight out of a 54# ILF set-up with the plunger set pretty stiff. and according to Stus thing I should be way weak I mean way weak. Shawn