The Hodgdon online data (maybe the same as Western) uses 500s as well. The pressures are in the 30k range, unlike the 5.7x28 which is in the high 40s, but also uses pistol primers, WSPs in this case.
Soldier on with you magnums IMO, but start at the start and work up (like we all should). If you have a chronograph, stop when you get to the top velocity in the data, or the top charge, whichever comes first.
Good luck with your project. I’ll soon be starting my own with the 5.7, as my cartridge gauge arrive today, the final piece to the puzzle.