Just use a bullet that's about 75% of the regulated bullet weight and the regular powder charge. For one of my 450 NE's they're set with 480 grn bullets. But 500's work fine also. 75% of 500 is 350grn which works out just fine. You can fine tune from there to suit your rifles needs.
Cal Pappas has used 75% of the powder with a full weight bullet.
I haven't tried that combo yet.