As long as you match some arbitrary dMin and dMax between your target densities and measured densities, PPEv2 is already set to do this type of calibration (has been for several years actually since the very beginning of PPEv2).
Simply put the target film densities (however many you have 21->) in the target measurements starting with the top density at A1 going down.
eg:
2.91
2.44
2.32
2.13
…
Then put your measured film densities in the measurements part. Ideally you have something like the 21x16 or similar target that you print for densities as this will give you a good averaging and more accurate density reads (a Barbiere or i1Prov3 may be useful here).
Just make the top density and bottom density =. So if the target density ramp says 2.9 for spectral white and you measure 3.2, just set it to 2.9 as long as the next down is less.
Anyone with a good 361T UV densitometer can paste their densities (that are validated through the normal precise PiezoDN process) here and others with the same densitometer can get quickly there (almost).
warm regards
-Walker