You need to measure your brass.
For best results you need to use a ball or tubing micrometer. It don't have to be expensive. I got mine off eBay for $20 some bucks.
Read this thread.
http://www.6mmbr.com/casenecktools.html

For example if you have brass that when measured the neck diameter is .010 you multiply this by 2 so .010x2 = .020 . Take that .020 and add your bullet diameter to that. (6.5 MM = .264) .020 + .264= .284 this is what your loaded round should be. Then for your bushing size take the .284 - .002 and get .282 this is your bushing size.

Now if your loaded round is measuring at .291 then your brass neck thickness should measure at .0135 or 13 and 1/2 thousands. (.291 - .263 = .027 divided by 2 = .0135) Bushing needed would be a .289

Now for optimal fitting if the chamber measures at .298 you will need .002 to .003 minimum Clarence between the round and the chamber for expansion. (Old wives tale is benchrest shooters like .001 Clarence, fact is with chambers that tight it causes unexplained fliers and other problems.) So with that said, You want your loaded rounds to measure at .296 to .295. So to get that loaded round you need the necks thickness of your brass to measure .016 to .0155 thick. (.296 - .264 = .032 divided by 2 = .016) I would say this is a no neck turn chamber.

The fact is if you brass is undersized it will not affect accuracy, but it will work the brass more, and cause it to live a shorter life. I doubt that your are sizing the brass with .009-.011 neck tension, I would say your brass has a smaller neck thickness that your are accounting for.