If it were me I would start with 40 gr of XMP5744 and work up to not more than 90 grains, behind a 500 or 600 grain bullet. XMP5744 is stable of a very wide range and you can pick a low point and work up. I have done this with several rifles including the .500 NE with 500 gr lead bullets, the .585 nyati with 1200 grain lead bullets, and the 470 NE with 500 grain lead bullets.