You are correct in that 6X would be 8 moa.
12X will be 1/2 the subtension (strata) so therefore 12X = 4 moa.
That being said, it would stand to reason that 18X is 3 moa, but it doesn't quite work out that way. 18X will actually equal 2.7 moa. 16X will equal 3 moa.
Here's the equations:
24/12=2X
2x2moa=4moa
24/16=1.5X
1.5x2moa=3moa
24/6=4X
4x2moa=8moa
If you want your spacing to be similar or very close to mils, then reverse calculate and you come up with 13.96X to get 3.44 moa spacing. 14X will be really close.
24/13.96=1.719X
1.719x2=3.438 moa or 1 mil
Test this on a yardstick at exactly 100 yds to verify the power changes are exactly as advertised. Some scopes that say 24X are actually only 23.5X type of thing.