David-

Nitro for black formulas for BPE cartriges are different than NFB for bore rifle/paradox loading.

Be Careful!

There are a couple of guys here on the site who own and shoot bore rifles and have been doing so for quite some time -- and have successfully worked a simple NFB formula for the bore rifles. This IS NOT the same as the widely published and popular 40-46 percent 4198 forumla.
It's completely different.

I own and shoot two different 16b double rifles, black powder, lubricated felt wadding, and roundball is all I use.
Why don't you want to run black powder in this paradox gun?

Will you please post some images of it for us to see?

I will search through my notes and see if I can remember who it is who has told me about the nfb for bore rifles formula.
I am concerned though that since your rifle is 28b (rather small for a paradox gun) and that it *is a paradox gun*, you're looking at stirring a hornet's nest with smokeless surrogate loads for it.
Black powder is so easy to clean up, so easy to load, and is exactly what your paradox gun was made for.
(please excuse my strong bias in support of blackpowder, I've had such great results with it in my bore rifles...)

--Tinker