If it was me and I couldn't get any reliable info. (try calling powder companies tech line, ie Alliant for RL22) They can usually get you pointed in the right direction. But if left to my own devices, Use of a reliable chronograph is a must; I would drop 5% below STARTING loads for regular 270 wby data (due to shorter throat in your
. See where your speed is and if any excess pressure signs are apparent. From that point (if more speed is needed)I would proceed to work up in charge weight (1 grain at a time) to my target velocity watching for signs of excess pressure along the way. It may not be kosher but thats what I'd do if I had no other information to go on.