That would be a very good load no doubt! The only fly in the ointment is Cordite is not available!

First I would slug the barrels to find out the exact bore dia. It is most likely a .411 dia, but you will not know unless you slug the barrels. When you get the rifle take the barrels off and look at the proof marks on the bottom of the barrels. It should be stamped with the proof load for Cordite. There should be a stamping like: 300 gr MAX This is the bullet weight it was regulated with. So if it was regulated with 52 grs cordite, and a 300 gr bullet, you can convert this to Re-15 by a formula of: Cordite load X 1.19= an equivelent load of RE-15 powder. with this powder there will be a lot of airspace left in the long case, so you will have to fill that airspace with enough dacron fiber fill between the powder and the base of the bullet so that the filler is slightly compressed when the bullet is seated. DO NOT LEAVE THIS FILLER OUT!

55.0 grs of varget with a 300 gr hornady rn bullet (.411) will yeald you 2079 fps.

What I would do first is buy a box of new Hoenady factory ammo and see how it regulates in the rifle!
.....................Good luck, and welcome to the DRSS