Given ideal conditions, I think 400 yds would be about the max you could consistently keep groups under 18". You're limited by both the cartridge and the rifle. Lever actions rifles are wonderful firearms, but they aren't known for top accuracy.
The .357 Mag is a fine cartridge, but an empty case only holds 27 grains of powder. Seat a bullet and you'll cut that in half. Not a lot of engine displacement to work with. Conventional wisdom says for long range performance you'll want to shoot the heaviest bullet practical.
Best case scenario your looking at a 180 gr bullet moving out at 1600 fps. Rainbow like exterior ballistics not unlike the American Schützen target rifle of the 1890s. Or the British Cadet/Rook rifles of the same period. The .38-55 being, perhaps, the most popular Schützen cartridge.
Of course, these old target rifles were all single shots, incorporating the finest workmanship and highest accuracy concepts of the day. They were fairly light weight and designed to be shot off-hand, those are about the only similarity they share with a modern mass produced lever action.