Rex,
Sorry that I offended you.
You can actually come up with a pretty good idea of functional BC out to 300-400 yards by chronographing a handload both at the muzzle (or 10 feet in front of it, depending on the chronograph) and then again at 100 yards (or 100 yards, plus 10 feet, then comparing the velocity loss to other bullets with different BCs. Probably the best way is to use a ballistic program, adjusting the BC input until the velocities match your results.
This is pretty much how many companies come up with closer-range BCs, because it's close enough for shooting at what many hunters consider "normal" hunting ranges. It's not valid for longer ranges, because BC isn't the written-in-stone number many shooters believe, but certainly would work well enough at the distances you mention.
