I am using 293 and 292 bushings in the 6.5-284 with lapua brass. Your brass may have different neck thickness however.
General rule of thumb is measure the neck OD of some loaded rounds, take the average of 3 measurements on each neck for 3 different cases then subtract .002 from that number...........Order that bushing plus one size smaller and one size bigger.
My loaded rounds are averaging .294, so I used the 292. But, I found I get less runout with the 293 bushing, so I started using it.
If you don't yet have a way to load a dummy round, you can measure the neck thickness, multiply it by 2 and add the bullet diameter. This will give you a very similar number to what your loaded rounds will mic at. Providing you use the proper tools to mic the necks..........dial calipers are not the proper tool, unless they have a sharp blade at the tips.