Ron, sorry if I missed it somewhere, but have you ever tried Trailboss in the 458? I believe a case full works out under 20 grains or so( dont take my word for it anyone, I dont have either powder or case to test, going off Prof Google here only).
Also getting into real rarified air, any thought on guard loads for the 458? Guard loads were the 8-10 grains of bullseye type powders in 30-06 with jacketed bullets for guarding civil enclosures in WW2. Velocities around the speed of sound, groups were actuallly pretty good 1-2 MOA, obviously very low report. Information probably of no worth for your project, no idea whether 'scaling up' to 458 guard loads is possible or dangerous ? but thought I would throw it in there anyway.
