Homer, I understand, you've come on to a new forum, want to make an impression of the guy in the know...but your reverse logic, just doesn't wash....
I suspect you yourself dont fully understand what and why you're doing it.... but by all means, keep on keepin on
If the bottom halves are of a finish that requires lapping..... it would kind of follow that the top would also...
But by all means, you do it the way you 'understand'....
Strikey, the panel beater's /diy/out the back paddock with a piece of fence wire, a hammer and your lunch (which conveniently includes a can of drink) - solution to your problem of no gap.... is to shim it.
[don't get too wound up Homer, I'm just stirring you up mostly
]
If you would like the considered response; the lapped internal of the ring would theoretically replicate the diameter and shape of the scope.... place a small amount of material, say under the top ring half, and you'll have a much reduced CONTACT AREA (defeating the lapping purpose) and force the tension screw to pull the ring so ever out of round...
Placing a longer shim to line a more significant proportion of the inside surface; will cause the inner diameter of the clamping area to be reduced, ever so slightly, while the clamped tube obviously remains the same.....will you be setting up random forces and stresses in the tube
hard to say and for a $100 scope I probably wouldnt be fussed..... but considering I'm a pedant, I'd fix the problem, not bandaid it.....I'd use the correct/engineering solution;
Take some material off the underside of the top ring half flat.
So, to sum up, my opinion, FWIW about lapping only half the rings is ...