Yes sorry, I was multi tasking when replying and made it quick. The lip of the 18 wheel is about 21/64" further away than the 20.
1/2" difference in the widths, divided by 2, plus the 2mm (5/64") offset.
But the tire is still 12.5" wide on both the 20 and 18, which negates the wheel width difference, so the edge of the tire (which is what I should have specified, sorry about that) is still only 5/64" further away. And the inconsistencies in tire manufacturing and the deflection of the tire can more than make up that difference.
Basically what I'm getting at is there isn't so much of a difference, that I wouldn't not worry about if one or the other rubs, I'd assume if one does the other does.