I have owned,reloaded for,and shot,belted/non-belted magnums for quite some years. Some with the powder capacity of a 30-378Wby Mag,416Rigby,45-90Express,458Lott,and others.
I started out using CCI250's and Fed215's and just stuck with them in my 308Win,243Win,and etc loads.
I have found that about .3gr -- .5gr of powder equals the difference between non-mag and mag primers. Even the Fed215's. This,for pressures and velocities.
The above works for me. YMMV. --- SAWMAN