If each deck is ten feet, two decks would be at least 20-feet of the saucer rim... and there is a few inches between each deck. So when you take the thickness of the rim, then subtract 20-feet and a couple of inches... what's left over?
Why are you assuming that the decks on the outer rim are full 10 foot decks?
Other than near the dorsal, deck 6's inner core is completely divided from it's outer rim. You have to go to deck 5 to move from the outer rim of deck 6 to the center core of deck 6. The center core has a height of 10 foot, but the outer rim doesn't... nor does it have to as we never saw the outer rim (of either deck 5 or 6) in the show.
And none of this is a new subject... I refer you to the sketches I've done in my thread
to date. At no point do I attempt to shoehorn in two full deck heights in the limited space of the outer rim... which is already diminished by the saucer's under cut. There are about a dozen graphics in that thread all displaying the same thing... a shorten height for the outer rim of deck 6.
This is another thing that the 1080' length helps out with, by the way...
If you assume that Decks 6 and 7 don't have the full 10' "filming height" but rather a more reasonable 8'4" deck height (comparable to many homes) and go with the larger scale... it's not really a stretch to have two FULL decks there (perhaps requiring just a tiny amount of tweaking of the undercut curvature?). You get, instead of one deck and another "ring deck" out there, you have two full decks, with an underslung equipment bay
around the rim.
This, to me, is the only way that the "undercut" makes any engineering sense. It's not cutting into existing deckspace, it's blending between the bottom of the "plain" area of the deck-7 floor and the "underhang ring" underside.
Interestingly, this actually could sort of work with the "Abramsprise" idea... the "undercut" (which is absent from his model) was added in some later refitting.