Never got a .45-70, but I did get a .308.
15gr of Unique and it shoots wonderfully. I know lots of guys over on Silencertalk run ~6-8gr for subsonic loads, but I didn't have any need for that. Plus, 15gr fills about 55% of the case so I liked not having to worry about any double charges.
17.5gr shot just about as well.
20gr started to show sights of high pressure, but nothing excessive. it fills about 80% of the case, and is almost exactly half the density of my standard rifle powder, H335. Primers weren't terribly flat (about the same as factory hunting loads), but extraction was noticably sticker (though the cases weren't as clean is I could have gotten them). I didn't get to chrono it, but I estimate 2100fps out of my short (16.4") barrel with this load and a 147gr bullet.
I've read that you can do 25gr in a .30-06 case, but it will be at the absolute limit (about 60k PSI), and obviously one should work up to that. That should give about 2400fps with a 150gr bullet, or about 2700fps with a 110.
In any case, given .45-70 has about 40% more case capacity than the .308, it should be able to take at LEAST as much powder. In fact, I think a very safe general-purpose Unique load is to find the volume that will just overflow the case, and half that for a Unique load (with no heavier than middle-weight bullets). This is about 15gr with a .308, probably 16-17gr with a .30-06, and right about 20gr with a .45-70 case.
And btw, if I'm not mistaken, Unique was originally designed for reduced rifle loads.