I'm putting one together right now and according to some people that are shooting the 6-284 I spoke with as well as Quickload data Ramshot Magnum looks like a great powder to try. That is what I am going to start with.
I am having a 26 inch 1-8 twist benchmark installed onto a long action rem, .900 at muzzle with flutes! I was looking for 3300 or so velocity with at least half MOA accuracy with the 105 weight bullets. I have Ramshot Mag, I will have to give that a try, I think that powder is temp stable!!gun)
Better check this against some trusted load data but I think my wifes 6x284 is running 55gr of retumbo with 107s. I have not chronoed it but it shoots pretty well. It shoots much better with the 105 lapuas but they are not supposed to be good for hunting.