Any Shot You Want, the A-Square reloading manual, shows that 70.5 grains of Reloader 15 powder will drive the 400 grain A-Square bullets to a MV of 2254 fps. 75.0 grains of the same powder is good for a MV of 2394 fps, at a reasonable MAP of 48,800 cup. Load to a MV of 2350 fps and the trajectory will be identical to the factory load. These loads were developed using A-Square brass and CCI 250 Magnum primers.
On reloadersnest I found a load for the 400 grains Barnes X with 64 grains Reloder 15. I think i will use that as a starting point for the 400 grains Nosler Partition and work up from there.