WAG, i would start with data for 35 rem in lever guns, and do a volumetric comparison of the two cases by filling and dumping each type on a powder scale.
http://www.hodgdon.com/data/rifle/35rem.php
the 303 throws a 190 grs bullet at about 2600 fps. hot loads for 35 rem in bolt guns will do the same with a 200 grs, but should not be used in lever guns. moderate pressure loads should acheive this in your gun.
what i think you will find is that the british case has more capacity, but most guns for it will be converted enfields, and designed for 48000 Copper Units Pressure (?), while most bolt gun 35 rems are designed for 52K CUP. long and short of it is that you prolly can't go wrong using data for 35 rem, but will get lower velocities (and pressures) because of the greater case volume. i always liked hodgdon H335 or BLC2 for this kind of application. if you are willing to interpolate, you are my kinda guy. do an interp on the basis of comparative case volume, back off 5 %, and let 'er rip, looking for flat primers or case head expansion (loosing you here?)
similar conversions of the 30-40 krag to 35 cal enjoyed good reputations in the US in the 1910-1950 period. grandfather had one of these. a very nearly identical case interms of ballistic potential, operating at nearly identical pressures, with nearly identical results, loading data almost interchangeable.