If you're willing to spend $300 on software for a PDA/Pocket PC, Horus Vision's AtragMX does exactly what you are asking for. BC is dynamic, so it does change throughout the bullet's flight based on velocity. A big problem is everyone uses the G1 (C1) ballistic model to compare all BCs. The G7 model is designed for long caliber, boat tail bullets and matches the curve associated with the long, slender, efficient bullets used for long range shooting, unfortunatly, it doesn't compare proportionally to the G1, and is a much smaller number, so to make a bullet look good manufactures use the G1 coefficient. AtragMX allows for input of multiple G1 BCs along the flight, and the software will even calculate it from empirical drop data gained from your rifle. I love their product, but without accurate environmental and range inputs, crap in=crap out. So get yourself a kestrel 3500
or higher model and good LRF while you are at it if you don't already own them. To solve your velocity problems: Many powders are temperature sensitive. Temperature changes the burn rate, which effects muzzle velocity. AtragMX will also keep track of different velocities for different temperatures, eliminating this problem. So you will want to test the velocity of your loads for different ambient temps and record your average. I have no affiliation with Horus Vision other than being a satisfied customer with their products.... I only wish their customer service was better.