Try these, the Gold Tip website FOC calculator: http://www.goldtip.com/calculators/foc.asp

or go to Jackson's Archery Page and try this one here: http://home.att.net/%7Esajackson/foc.html

Jackson's page is great, with a wealth of resource info, including a pretty awesome ballistics calculator. It's been in my "Favorites" for about 3 yrs. now. Good Luck, hope that helps a little.