The 1 grain velocity/pressure factor is Richard Lee's concept, and he gives it in his book (at least, it's in the 2nd edition) for which he had the opportunity to measure it for some bullet and chambering and powder combinations. Obviously he didn't have the opportunity to do it for everything.
I don't know any other source of that number. You may be able to deduce it from published pressure and velocity data. I haven't paid close enough attention to it to try. I use
QuickLOAD and a chronograph to get a good match between it and my components, then let it predict the pressures for load changes. Usually works pretty well.