Gordon’s is a good tool, but you have to be very careful with it. Results from it can vary wildly, and some of the powder models are not very good. They stopped updating it when he died, so it’s missing all the newer stuff. Cartridges and bullets are fairly easy to add, powder not so much.
The variability of the program is well evidenced by what happens when you copy recipes from published reloading manuals. A published load might be right on the money in the sim, but I’ve also frequently seen them vary from 40ksi all the way up to 80ksi in the sim. Things like measured
case capacity, bullet initial pressure, boat tail length, etc can all impact the sim. A bullet initial pressure change (from one standard value to another) can bump the sim 3ksi. Ditto on a grain or two of case capacity difference.
I like to start my load dev for Gordon’s data around what they list as a 44ksi load, and work up from there. That leaves plenty of overhead.