Re: Vihtavouri N540 or N550 powder
N540 for up to 190s in some barrels, n530 in others and n550 yet in others. I'd start with 540. I've run a lot of 540 and 550 in 308s running 168s up to the 208s. 540 has been stellar in both accuracy and velocity for 168-190. 550 is better for the heavies.
Long range shooting is a process that ends with a result. Once you start to focus on the result (where the shot goes, how big the group is, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.