Loki, there are doubtless ballistic formulae to help one figure this out, but if you have access to a chronograph, or don't mind shelling out $100 or so for one, it's a great tool to use. I've found that some printed velocity figures are almost spot-on, yet others in the same books or charts are quite a ways off from what MY individual gun produces..
I don't have the data here at work, but out of a 20" barrel you should get about 1100 fps with that load.
You can actually load a .45 Colt case up quite a bit hotter for a good lever rifle such as Marlin or Winchester 94, or a Winchester 92. If you want some good info, check out Paco Kelly's article on this topic at leverguns.com. Paco's stuff on leverguns has no peer in the gun literature.