I was thinking of trying out reloader 25 in the 600 NE. 159 grains of Re22 gives 2150 fps with a 900 grain bullet, but I was thinking of going up to Re25 to have a slightly compressed powder charge. There is a bit of room in the case under the 159 grain charge of Re22.

Any suggestions where to start with Re25?

Try IMR 7828. I had pretty good luck with it in the .600Ok with 159 grs of that powder and just a Fed 215 primer. No ignition problems at all.. I hit 2150 with no pressure signs in Lathe turned brass. The optimum powder should be H414 though! No experience with Rl-25.-Rob

The RL25 is enough slower than theRL22 it may fill the case like you want.Start with the 159 gr you had the RL22 at and work up.Ed.

I ran some numbers through Quickload FWIW.
Your load of 159gn Rl22 showed 2013fps @ 32.5ksi
RL25 159gn - 1935fps @ 28.7k
165 - 2018 @ 32.0k

I have no experience here, but would think that the 165gns of Rl25 should give you close to the same velocity as your 159 Rl22 load.