I go the opposite way and just change the RPM in the formula until I get to the surge line, probably make more work for myself but I can gauge and plot that way how efficient the compressor is at a range of RPM's. Just to add, do take the numbers you get with a pinch of salt, don't expect a 1.05 AR turbine housing to spool the same as a 48 AR housing just because a compressor map says so lol.