I have used up to 90 grains RL25 with the 300 SMK. You can probably start at around 86? for the 250. I like RL22 for the 250 though. Do a search--if I remember there is a member who goes by Guy (user name) who posted a bunch of data. Also there is data at www.accuratereloading.com
--these guys really load em hot so be sure to work up.