I could not find any info in the manuals for the RL 25 / 212 ELD-X for the .300 Win Mag so I did some searching and some number crunching and made seven 3 round groups of RL-25 from 73.5 to 75.9 grains. Shot over a chrono, and #3 group had good ES / SD and .4MOA, and 75.9 grains produced a velocity of 2,940 to 2,950 fps, some primer flattening from middle charges up to max. Groups #3 to #6 was under .5 MOA, its acceptable since its primarily used as a hunting rifle.
Is 2,950 fps with a ELD 212 grain bullet way too fast? I have a 26 inch barrel, and weighed H20 case capacity
( 94.78 grains ) and data into Quickload and its a hair under the red line. I assume the 200 grain bullets are around 3,000 fps and 210 grains at 2,875 to 2,900?

Since groups #3 to #6 has under .5 MOA, which charge do I look for? The one with the best SD / ES and play with seating depth to tighten a bit more? #4 to #6 has same POI by the way.

sounds like its coming along nicely

That seems fast... That being said, guess I'll be picking up some 25 to push my 200 grainers ELD-X's.

As far as where to go from there. I wouldn't use the max load, middle load with a tight SD.

Went back out to confirm loads, seems that the best load so far is at 2,925 fps @.4 MOA, it was backed off about half a grain. Tweaking seating depth may tighten it up. Saving that load for nilgai hunts soon. Working on some 180 Accubonds as of now with some RL 22

Shot the 220gr ELD-X out of my 30-378 at Nilgai right at a month ago...got. 700lbs bull 1/4 to me at 230 yards and put it inside the right shoulder and had a exit right behind the opposite. Bull bucked up and took 10 steps and was dead in his track. I shoot that ELD-x with confidence in several rounds.

That is a great velocity with a spool heavy bullet.
The long barrel is going to give you more velocity.
Every gun will coronagraph different.