I tried a variety of loads and seating depths with RL-22 and IMR-4831 but the RL-17 shot best in his application. All seating depths tested were set at max magiziine and shortened in .020 increaments with mid loads until a nice group showed up. Then charges were increased in .07gr increments until good numbers or pressure showed up.
Start your testing below my final charge numbers and work up with traditional safe practices.
Use the max load listed on the accurate website for Magpro. It is the best load that I know of for the 300 winchester. My 24" barrel rifle is shooting the 180 grain Cutting Edge bullet over 3300 fps with no pressure signs. My accuracy load was one grain over the listed max. I have shot 300 win mags since 1969 and this is the best load I have ever shot. Have never got over 3200 before.
Predictions are difficult, especially when they involve the future
I have a great load that works in my M77 MII all weather ultra lite, its Norma brass, CCI BR2 primers, 73grns Imr 4350 with a OAL of 3.487" .030" off, shoots 3116, slight resistance with bolt lift, this load is way over max book, 67grns is a good start.