To figure BC of a bullet you have to shoot the rifle at known distances, record the drop at each distance, lets say your sighted in at 100yds, then shoot 200, then 400, then 600 and so on to 1000yds. Compare the programs drop for each distance, adjust the BC for the bullet(through the program) until it matches the drop you recorded. This will be the actual BC for the distances your shooting. I used the 300MK sierra in my 338 Snipe-Tac rifle, .768 is what Sierra states, Through my testing and recorded drops my Exbal program came up with a BC of .813 (3325fps) this was shot at an altitude of 1200ft. Its a time consuming process, but can be well worth it to fine tune your program.
I forgot to mention you also need to chrono the loads at each distance to match up the speeds. This can be scary for some, boom goes the chrono!