Re: 338 / 300 GR B.C.??
Method 1) Use two calibrated chronos. 1 @ 10 feet the other at 200 or 300 yds. Then plug the data including all atmospheric data into an external ballistics program and vary the BC to make the velocities fit the actual.
Method 2) 1 Chrono for MV. Then zero @ 100 and shoot at 200, 300, 400, 500 and 600 yds to measure drop at each distance. Record all atmospheric data.
Then vary the BC input to the ballistics program unti it fits.
Things get fairly confusing but you'll get close.
The published BC for the 300 smk works pretty well up here at 4450' MSL. Down where you are it will probably be a bit lower. Maybe lower 7s or high 6s.
I may be the slowest guy on the mountain . . . . but . . . . I'm on the mountain!