I consider the top tapes to be the "belt line" tapes. Measure around where your belt goes for them.
The lower tapes are the problem. They will need to cover more distance because your hips get wider below your belt. ALSO the wing in the front is having to curve out and around to meet the tapes, so it will lose a few inches there also. THIS is why most adult diapers are sized wrong. They measure what the top tapes will fit, and then they call that the top end of the range. There's simply NO chance of the lowers even reaching the front at that point.
So you have to know how "honest" the numbers they publish are. If you have a 38" waist, you're going to need around 42-44" around the lowers. So someone with a 36" waist thinks "oh this diaper says it fits up to 38, it'll be close but it should fit with a bit to spare". And no. Not even close.
Length seems to be something of a matter of preference. Considering how far the top tapes are from the top of most diapers, usually around 1-2.5") that places the top of my diapers usually between my belt line and belly button.
I prefer the padding to get to my beltline if possible. Some diapers that's a problem because they have so much "dead plastic" on the front end. Sometimes I cut some of that off. (tena slip maxi plastic for example) But that shortens the diaper which leaves less of a gutter down below. Works okay for day, not so well for night.