I use this ballistic calculator: http://www.handloads.com/calc/

You will have to check the velocity with a chrony or "guess". Most of my loads are pretty close to a 100 gr blackpowder/pyrodex equivalent.

BC for powerbelts are as follows for 50 calibers:

245 gr. = .161
295 gr. = .186
348 gr. = .220
405 gr. = .257