Antlers:
For maybe 30 years, the modern American equivalent of the 60gr cordite 'factory load' for the .400 3-inch or .400 Jeffery has been 80gr IMR4831.

These days, I would recommend the Hodgdon 'Extreme' equivalent, and others here will no doubt recommend a Reloader powder charge with some kind of filler-wad. Magnum primers are recommended so no dramas with the Fed 215s.

Any 3-inch chambered .400 rifle should regulate with this load, especially if Woodleigh bullets are used. Because of up to a century of wear and tear, however, some original vintage doubles may require tweeking the load a little for best regulation.

A modern rifle like the Heym should come with recommended loads, but I doubt they would differ significantly from the above. Newguy who posts here should have the information at his fingertips. No doubt he'll chime in shortly.

Hope this helps.