"We’ll just have to agree to disagree on the use of crescent buttplates." Fair enough!
I have no axe to grind on butt plates and I care little how shooters use their rifles. It does pain me though, to see a pretty and useful stock butchered because of recoil fear BEFORE giving it a try with a proper mount.
"The problem with the 45/90 is the lack of published pressure tested loading data. There is plenty around for the 45/70 but high pressure data for the 45/90 is hard to find. " All true - not easy, but since when are easy tasks very rewarding?
However, given the same bullet and load, the longer case of the 45-90 should generate lower pressure than the same load in a 45-70.
This load/pressure can be a safe starting place for .45-90 loads.
The Winchester .458 magnum case is 1/10 inch longer than the 45-90 and holds 8-10 percent more volume, establishing an upper limit for .45-90 loads.
Given these two points , even I was able to "guesstimate" where to begin and end.
There are also some good printed ( Authors like Sharpe and like Ken Waters + the Lyman Manual ) sources and online sources of 45-90 load data. The research is part of the fun.
With this amount of reference data on 45-90 loads, any experienced hand loader should be off to a running start loading for the 45-90. Naturally, if said hand loader is unable to read or use a search engine, he may use a custom load service of which there several in the USA.
PS, I also like the 45-70 and have a Beretta .45-70 double rifle and am helping my son pick his first 45-70 lever action. My help is him shooting some light loads in my DR and an 1886 45-70 - he did well and liked making big holes in the targets.