The reason you don't find much load data for the 45 Colt using Red Dot is, it's just not a very good powder in that big case. Just guessing but I'd say 6.5-7.0 grs just might be as far as I would want to go with a 250-260 gr bullet in a strong Ruger Blackhawk.
Recently I have been helping the widow of a very good friend get rid of a lifetime of guns & reloading stuff. In there was some 45 Colt loaded with Red Dot but I couldn't find any Red Dot powder on the shelf, I don't think my buddy ever used it & neither do I.
The load said, 255 gr bullet & 10 grs of Red Dot!!! Then I knew he didn't load it. He had a neighbor who had died 2-3 years before him & that guy had given him a bunch of reloaded ammo, I'm sure it came from that neighbor.
So I pulled it, luckily! 91 rounds had 10 grs of Red Dot, which is an overload for sure. I think 40 rounds had no powder at all & 52 rounds had 20 grs of Red Dot, yes a double charge! Enough to easily blow up any 45.
Dick