Like he said...I try to get 75%-80% contact. so if you have about 75-80% silver instead of black that should be your contact area. To get the talleys loose on a scope either the scope would be under sized or you spent several days lapping your rings
. They have a pretty large gap in them when torqued properly so not to worry about taking out too much material. Also as long as you leave 20-25% of the black surface you will be fine.
I never lap the top halves though always was afraid of pulling the threads out of the bottoms since they are screwed into aluminum. But that doesn't mean I am right and others are wrong, just how I do it.