Ah Excel, my favorite MS tool.
I'm in the process of the conversion, easy math you can do in your head.
Since the OD reduces the RPMs by ~30%, just take 10% of your current RPM, multiply by 3, then subtract.
Say you turn 3000rpm now,
10% of 3000 = 300
300 x 3 = 900
3000 - 900 = 2100 rpm
All independent of tire or gear ratio