The fact that a BC changes (decays) as it flies downrange is an unfortunate consequence of the BC being referenced to a non-representative standard (G1). If you reference the BC to a more appropriate standard (like G7 for long range bullets) the BC doesn't change (significantly) with velocity.
In order to use a G7 BC, your program has to be able to accept it (some programs only work with G1). I don't know if Michael's program can use G7 or not but there are plenty that can including this one:
*email me if you have trouble with the download (firstname.lastname@example.org)
Here's an article that explains more about G1 and G7 BC's.
Berger Bulletin Blog Archive A Better Ballistic Coefficient
The truth is that we shouldn't have to deal with BC decay; there's a better way.
Berger provides G7 BC's for our bullets on the webpage. I've also measured and published G7 BC's for ~175 LR bullets of all calibers and brands in my book
To answer your question directly, there is no way to reliably predict the G1 BC decay rate.