Roy,
I put together a .416 Remington Magnum around 1990, before Varget appeared, but would suggest trying it in your rifle. The powders that worked best in mine were around that burn-rate range, such as IMR4895, IMR4320 and IMR4064. It's also far less temperature-sensitive than RL-15.
Hodgdon lists 77.0 grains as maximum, but with 400-grain Hornady bullets. TBBCs tend to generate more pressure (and hence velocity) than typical gilding-metal jacketed bullets, so would watch the chronograph while working up. The velocity they list for the Hornadys is 2407 from a 24" barrel.
