Maybe I missed the multiplication by two for the two possible incrementations, so maybe it should be:
s = (nynx + nxnz + nxnz - 2) 18
The issue is a typo, which is not surprising with those crazy numbers I got. Above is wrong, below is correct and is working in my tests.
s = (nynx + nxnz + nynz - 2) 18
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