Trail Boss, 19 gr. (a full behind the bullet charge), Lee cast 405, or 20 gr. with the Lee 340 gr. worked for me, YMMV. I started with the S.F. Trapdoor loads and worked up, these are NOT their data and are from use in my H&R SS UH, 24" barrel. My impressions are purely subjective.
These two loads feel to me to be less than a full Fg black powder load, but the pressure rise puts this powder beyond BP loads, so it probably isn't recommended to exceed their Trapdoor level load in a Trapdoor.
FWIW, I've noted that some accuracy tests with the 45-70 with lead bullets have the best loads around the 1000fps range, a place that Trail Boss can easily achieve and have great load density with no powder position issues.
I'm trying out TB more all the time in several calibers and really liking it.