http://www.backcountrybowhunting.com/articles/tools.php I just ran my Xforce I'm getting I put in the IBO which is 342. the arrow he shot was 417 grains, and then he ran it though and it was 275fps, now if you punch in 29 inch draw and and 66.2 then it reads 304 fps.
he ran a 350 grain arrow it ran 292. now the website said 324.4fps...... Its kinda hit or miss I guess I changed the IBO to 310 and got the numbers to balance out to what the real deal is so.