2F vs 3F Velocity
 

Hodgdon's site only gives velocity for 2F powders with PRB. I think I've heard somewhere that as a rule of thumb you should expect similar results as 2F with 10% less powder. Ie similar velocity with 90 grns of 3F vs 100 grns of 2F. Is this about right?

About right.

The actual difference between the two might no be worth it. Also make sure to check because some rifles really like 3f and others hate it. But I normally advise that when you switch from 2f to 3f.. act like your working up the load for the first time. But start 10% less then the old load for your testing.