Right there on the barrel flats......70 Cordite 480 grain bullet would be the standard load that it was regulated and proofed for.
One can certainly try NfB reduced loads if for no other reason than less recoil.
If you do not have Graeme Wright’s book “Shooting the British Double Rifle” 3rd edition, it is surely worth it for a double rifle enthusiast.
Suffice to say for a modern powder load you can start with 1.19x the original Cordite load using Reloader 15, magnum primer, and either Dacron or backer rod filler or you can try 1.33x using 4831.
The same load of powder and 350 grain bullets will usually come close to regulating as well, they did load those and 400 grain bullets as an option for thin skinned game in the old days.