Quickload has a 22-250 reaching 50,000 psi with 37 grains of rl-17, and a 55 gr horn FMBT with a 92.62% burn efficiency from a 24" barrel.
Given those parameters, I'd probably start in at about 35.5 gr. .
But then again, I have a large box of brass with loose primer pockets that are going to be sold for scrap one day