After getting a couple of tips from some helpful forum members I went out again yesterday and shot some rocks with the 375 DCM.
We shot again at 2910 yards & then decided to have a go at 2065.
The 2910 backdrop is quite angled which doesn't give the best feed back from strikes, couple that with rain earlier in the morning & it gets difficult.
At 2065 yards I got some pretty small groups & hit the rock with both the 425 Cutting Edge & the 414 GS Custom projectiles.
I think getting the shooting mat out from under the rear bag has helped, thanks Broz
My biggest handicap is still the position, 19/20 degrees up isn't great with a tall bipod to do it.
A portable shooting bench should increase my consistency.
I use G1 1.05 for the BC with the 425 CEB & 0.82 with the 414 GSC at 3140 fps these work out at right for me at 2065 yards.
The difference in BC may not be so great at shorter range or with a slower twist barrel, these are numbers that work for me.
Here is the video.