Have your friend look at the loads on the barrel flats that the rifle was regulated for.
Example: My 450 No2 was regulated for 75 grains of Cordite [Tropical load, your buddies double may state 80 grains cordite]. Take the Grains of Cordite and multiply by 1.19, ie 75 X 1.19 = 89.25 grains of RL 15.
Drop a few grains for safety and I would start with, say 87 grains. Work up watching for pressure untill the rifle shoots good. My rifle shoots good with 88 grains of RL 15 and 480 Woodleigh soft/solids, 500 Hornady soft and old solids, and 500 Swift A Frames.