I find it a lot easier just to go here:

www.bowjackson.com

and type in the numbers. Lots of other excellent calculators on this site also. Hope this helps. Be safe.
Shoot Straight
Derbytown <img src=icon_smile_big.gif border=0 align=middle>

“An archer wants to see how far away from the target he can get and still hit it. A bowhunter wants to see how close he can get to his target before he shoots.”
Dr. Mark Timney