No...not the average of the loads, but an average of starting charges.

Take the average of a couple of powders on either side of yours. Average the starting loads and work up from there.

Ie Take BL(C)-2 and 748, maybe h335 and average all 3 starting loads to get your base.
“It isn't so much that liberals are ignorant. It's just that they know so many things that aren't so.”
- Ronald Reagan