I should think H4895 or Varget (a bit slower) would work OK with 300gr. and 350gr. bullets.

The only way to find a load (that I can see) will be to start with 400gr. loads and go up a couple gr. at a time form there.

I suspect you'll be shooting close to a full case of powder(maybe even compressed) by the time you get to anything resembling maximum pressures with the light bullets. The ballistics should show better than .416Rem by the time you hit descent pressure for the #1.

I also think the projected 2,400fps is quite conservative, afterall, how does the case capacity compare with known .40's? I am thinking of the .416Taylor, .416 Remington, and the .416 Rigby. Where does the .450/400 case fit?

Some load extropolation could be worked out from known data, especially when you use H4895 or the ADI quivalence as H4895 can work down to 50% reduction with total safety.

Some powders can become dangerous when reduced. This makes H4895 a great powder for load development as it's safer to work with and ignites easily in less than full case loads.