If you are adjusting COL to get the bullet closer to the lands, I presume you're doing that to improve accuracy.

As Fishinfool said, there's no formula that will tell you how much velocity you may lose (via less pressure) or perhaps gain (via increased pressure due to proximity to the lands), and no formula to tell you how much powder to add or subtract to compensate.

Changing the seating depth in this way *will* allow you to physically add more powder. That may or may not allow for more velocity before pressure signs show.

But the way you find out is by working up the loads and testing seating depths & powder charges vs what your goal is . . . velocity or precision.