If it's ballpark you want, you are already there!!! I have talked to a few wby guys and Ack imp. guys, and RL 25 seems to be the powder of choice by most.....not all.....but enough to encourage me to try it first on every new bbl I get.
Also, I don't know if you can shoot a match while FF the 300 AI......if not, save your self some cash by shooting cheap bullets during the FF process. Or try the fast powder (with or with out the cream of wheet) and don't shoot any bullets. Some swear by this method, some swear AT this method. I have never tried it on a big case, but it worked out very well with my Dasher.
Again, good luck!
Tod