Some German data may be helpful: The .35 Winchester is very similar in volume and other respects to the shorter, but fatter 9x57R.
An older, 1991, German proof table lists copper-crusher average maximum pressures for the .35 Win as 2700 bar, the 9x57R at 2500 bar and the .303 Brit. at 3200 bar.
The newest 5th edition of the DEVA book "Wiederladen" uses the electric transducer pressures of the 2000 proof tables. These do not list the .35 Win. anymore, but the .303 @ 3650 bar maximum, the 9x57R @ 2800 bar.
For the 9x57R it lists these pressure-tested loads, all giving a .358" 250gr Speer bullet 2200fps: 46 grs Norma 202 = 2150bar; 46grs IMR 4895 = 2386 bar; 47 grs VV N140 = 2341bar.
As your RL15 is slightly slower than any of the DEVA listed powders, your load should be well inside the .35 Winchester limits, even in a rifle that old.