RL-22 for the 6.5 rem mag

[Blacktail]

I have a 6.5 rem mag built by selby on a 700 titanium action scoped with a 4.5-14 leupold come in at 6.5 pounds. Just about to start loading for it. A buddy g.ave me some 129 sst so figured I give them a try I have no exp with this caliber. I have hear rem 9 1/2 mag primers and RL 22 I have both but curious where to start in terms of charge. I am concerned the ssts may not shoot well do it being a long bullet and its a short action. Any help greatly appreciated. Hoping to get around 3100 fps. It has a 22" shillen and is 1:9. Going to be a pack gun for blacktails and occassional mulie if I dont take my stw.