I just opened this forum to start another thred on bushing dies and saw yours. I am just going through this process - first time bushing die sizing... for a 300 RUM.
I wanted to order the dies and bushing, but had ne way of no how large a bushing I needed (and you really need to turn the necks for this process as already explained). Sooo.... I took a couple of pieces of brass and neck sized them with my 300 WSM
die so I could turn them. I turned them then measured the OD. Then I seated a couple of bullets in them and measured the OD again (this is the important measurement) I subtracted .001 (for .001 tension) and came up with .333 for a bushing diameter, which BTW, is the same OD I get if using the expander ball.
I loaded up 50 rounds yesterday for 5 different powder/bullet combinations for max load testing. With 180 gr E-Tips seated at 3.690 I ran into compressed loads with 99-102.5 grs of Retumbo. The .001 tension was not enough to hold the bullets down with the higher compressiojn. I had to re-cam the lever several times to get the bullet to seat.
I'll be asking some other related questions in my thread.