duanedr 

You don't say why you're measuring this or what precision means in your context but to increase the accuracy of your measurements you could turn some chunks of tube/bar (delrin, alum, etc) in a lathe to slightly oversized for each socket's tube size. Then mock up the joint as GrayJay suggests. This will remove most of the slop and give a more precise fit and measurement. I would add to Ti's comment that the socket will have 1-2* of 'slop' and it's pretty easy and common to manipulate the socket (up to another 2-4*) with some basic blacksmith techniques. I don't think I've used a set of lugs that didn't need some tweaking.