Have you considered XMP5744 powder from Accurate Arms? I have used it for reduced loads in several calibers including the .500 NE for below 1500 fps velocities. Probably if you contact Accurate Arms they will suggest a starting load. In my .500 NE I use 45.0 grains behind a 500 grain cast bullet. There is a huge volume of empty space in the case, but I have not used a filler and have had no trouble.