So, if the fans are pretty much touching it is 15mm. If there is some space it is 20mm.
no this is wrong because your ignoring the diameter of the fan itself.
140mm fan has the fan opening of 140mm
That means its 140mm on each side facing.
The corners opposite to each other is 198mm +/-, hence why mnptech gave you a
20cm spacing for 140mm fan.
Had the fan been 120mm, you would need to do a 15 cm spacing.
And the cases you listed have
DEDICATED MULTI MOUNTS, for 120mm and 140mm fan.
They are not sharing the same hole, and again those holes are probably spaced out 15cm for 120mm AIO's 20cm for the 140's.
What your (im guessing implying) is the wider spacing Thermochill PA120.3's used to have which were i believe 18cm.
This gave you a gap between the fans, but no one uses this spacing layout anymore, and would require you to source old PA120.3's
Also i don't think your implying 15mm of spacing between each fan housing, because there is no radiator in existence which has spacing like that, and you'd be at the mercy of using only 120mm radiators.