I am not sure what the optimum shoulder width is, but I do know one thing it is good for that hasn't been mentioned, that I saw. The larger the shoulder the larger the bearing surface that mates with the suppressor. This is good as it helps to prevent the suppressor from loosening due to thermal changes or recoil.
As Boomer pointed out, this can be accomplished with a locknut or something similar, but then you are taking a chance that it will lock up out of alignment with the bore. I know there has been been a lot of back and forth regarding that, and I see where people have gone back and forth, but I think there is a fundamental misunderstanding of why relying on the threads being concentric to the bore will allow for good alignment. In order for two threaded members to actually thread together, there has to be some "slop" between the surfaces. Otherwise you would end up with some serious galling. That is why, when you are threading the suppressor on the barrel, before it locks up on whatever shoulder you provide, it has some wobble. Just using a jam nut, lock nut or adapter that is not set against a surface that is perpendicular to the bore means that it could still be out of alignment. Maybe not enough to cause a baffle strike when tight, but when added to the fact that we no longer have that bearing surface to keep it tight, it could become a big problem.
All of this goes out the window if the adapter is mounted semi-permanently (loctite, Rockset, etc) and then trued up with the bore, or permanently attached true with the bore.
Also, as pointed out, even if it is not true with the bore, it may not be a problem. If you are using a .308 can on a 6mm and it is 0.005 off of center, it is only going to be around 0.020" off at the muzzle of a 10" suppressor, which, as pointed out isn't even close to the amount of extra space available for the bullet to exit. It may affect your accuracy, it may not.
The one point I do agree with fully is that a 1/2" diameter thread pitch on a .308 barreled rifle will leave a very vulnerable spot in the minor diameter of the thread and the groove inside the barrel.
Last edited by HandgunHTR; 10/30/19.
