I think a lot of it also depends on your elevation and humidity. Your not gonna get the same BC at sea level as you are at 5000ft. Usually the higher altitude will yield higher BC. For this I cannot fully explain, just what I've read. Kinda sucks when your only 600-700 ASL like I happen to be...
I am shooting the 338 EDGE w/300g SMK at 2840fps and my charts weren't matching up real well from the start...From 300-1000 they were anywhewre from a couple clicks low at 300, to about 12 clicks or 3 MOA low at 1000. So either I'm not getting the .768 BC value, or my speed really isn't 2840fps,which I highly doubt either as they've been time tested by others on this board. Everyone seems to be getting .770-.800 for a BC on the 300g SMK, and I"m using the standard 92g H-1000 load that everyone else is using in the EDGE to get 2850fps, give or take a few FPS. So I just figure that at my elevation, which aint chit, the BC must not be .770. Thats about the only conclusion I can come up with. If I enter 2700fps instead of 2840fps, my clicks match up perfectly out to 1000 yards, based on my actual shooting, thats all I know...Never did work the BC out backwards, but I have my drop charts refined to my actual shooting and thats really all that matters in the end.