buckstix:
I had a very nice Alex Henry ULH double-eight in my gunsafe for the best part of a year a while back.
Didn't belong to me unfortunately, but I got it shooting really well with nitro-for-black and a 2oz round ball in the long case.
Here's the load FYI:

65gr Blue Dot, heavy card over-powder wad, then two and a quarter sheets of Viva paper towel ("kitchen paper" in the US?) rammed hard,
then another heavy card wad under the projectile. Crimp needs to be solid.
Initial efforts with a light crimp produced the occasional barely perceptible hang-fire.

With that load, regulation was perfect in that particular rifle. No guarantees in your Rodda of course.

...and congrats on acquiring this marvelous double eight. They were a milestone rifle in the history of Brit big game hunting, particularly in Africa, bridging the gap between the large bore muzzle loader and the nitro express.
...and Jack Lott's example might just be the most famous of them all in an American context.

Personally, I would kinda prefer Gerald Burrard's one...
Bit more Raj than Dark Continent!