The 350 RM and 35 Whelen have nearly identical case capacities (per Chuck Hawk's website):

.35 Whelen (R-P): 72.6 grains of H20
.350 Remington Mag. (R-P): 73.7 grains

Use Speers data for the Whelen and the 220 gr bullet that uses CFE 223 as a starting point minding safe load workup protocol:

https://www.speer-ammo.com/download...35_caliber_357-358_dia/35_Whelen_220.pdf