David-

In a conversation with Lewis Drake a couple of years ago on the topic of getting regulation loads together for blackpowder roundball double rifles, his simple advice was...

'start with xDrams of powder, keep adding powder till the barrels shoot together'

It's a good rule of thumb, and I think Gatsby's advice of a 1Dram sarting load is a good first try.

Here's a link to a conversation I had with some of the guys here a couple years ago when I got my second 16b double rifle.
Link to 16b rifle thread

It might take a bit of experimentation on your part, and it might also take getting balls of groove diameter.
What is the groove diameter of your rifle?

Also, if you're going to star or roll crimp your cases, make sure you start with cases trimmed for your chambers.
My sense is that properly loaded, you won't need a crimp. In the case you end up with balls near the end of the case, simply seal them with a little melted beeswax.

I use Goex FFg with great success.
I've tried other brands of blackpowder, but I find that the Goex burns clean and performs well in my rifles.
If you can't find Swiss, go with the Goex and see how things turn out for you.

--Tinker