Re: BC calc.\'s
I haven't looked at the accuracy issue for a two range velocity derived BC. It is pretty easy to get if you have a rangefinder and a chrono though, two chronos makes it even easier, and a tad more accurate if they're calibrated together. I think it's more accurate than the Oehler 43's acoustic target because the distance is less critical, much less.
The most important thing for me is simply to use the one that matches my fired drops, I know what the MV, temp, alt and BP is, so BC is the only other variable that will modify the trajectory.
Once I establish this baseline as my sight in perameters, the program is used to modify them as the other variables change from day to day, air density being the MAJOR influence if you have a stable MV with temp insensitive powders.
After 800 yards you really notice the subtle MV or BC changes.