Wondering how you figure out the max range of a bullet while still supersonic. I am shooting a 7mm mag, 168gr Sierra Matchking, roughly 2890fps from muzzle. At what range will that bullet no longer be supersonic? Is there a formula? Thanks.

Run the numbers on the JBM website.
Google is your friend!

You can look in the back of a good many Manuals and you will find the External Ballistics Tables. Easy to use.

As long as you are above about 1080fps at Sea Level, you will still be above Mach 1.