If you follow the link in my post #9, you'll learn there is no standard approach to making magnum primers. They all make more gas, but some are higher brisance than others, some have long flame sustain, etc. So it is easier to do as JKP suggests with some primer brands than others. My rule of thumb for small rifle is to knock the load down 5% when changing primers, as that seems to cover the range of difference fairly well. I haven't done that yet for small handgun primers, so I would fall back to the 10% reduction until you know what you've got.
Use your chronograph to look for a change in velocity and not exceed the velocity you got with the other primer. If velocity goes up, then your pressure went up, too, and you want to back the load down.