It appears to be somewhat conservative. I ran simulations for 10mm Auto, 180 grain Speer GDHP, using the three powders Alliant lists in it’s data: 2400, Blue Dot and Unique. In all three cases the Alliant max charge was reported by GRT as overpressure by about 2K PSI.
I plan to use this to try to get a better handle on some of the non-canister powders I have, as well as better guidance for some of the cast bullets I use. Having GRT’s data be conservative is a benefit.