The only relationship the 45-70 has to the 10.5 X 47R is the latter can be made from the former. Data is not interchangeable. Bullet weight for the 10.5 probably shouldn't exceed 300 grs., if that.

fiat, do you have Grahame Wrights book on "Shooting the British Double Rifle"? Ballistics care not for country of origin and I found his book most helpful when I got my first double almost 30 years ago.

If you have difficulty getting enough velocity with straight black powder, lightly duplexing the load with about 5% of SR-4759, 2400, IMR-4227 will usually remedy that difficulty. It will also bur amazingly clean. What I found amazing was that most lightly duplexed loads produce less than proof pressures of straight BP. Wright's book covers them in detail.

IF you can find any or, have it on your shelf, Trail Boss might work ok but I doubt it will regulate. I've never known anyone who tried it who got it to regulate. It always shot apart. As far as BP's, Swiss or Olde Eyensford will be your best bets. I have heard the new GOEX is pretty good but I haven't seen any, let alone tried it.