I cant help with the light bullets in a slow twist. Mine is/was designed for 55 grain Bergers in a fast twist ...........shoot me a pm if you want info.
John
20-250 26" pacnor 9tw
55gr berger 33gr IMR 4166
3470fps
Hoped for faster, but pressure builds fast in these over bores. Shoots very well. Easy on fur too. Last year I killed a fox from 650 bang flop
I would start with starting data from a 22-250 and 45-50 gr bullet. Pressure may be higher than starting due to the smaller bore, but with current manuals with reserved numbers and a quality action, should be a good starting point.