Pretty sure that the rule of thumb for mm to fractional scale is what I think of as "The rule of 1800".
I think this is because 1800 mm is roughly 71 inches which is a good easy standard for an adult human male height ?
In any event, multipliers yielding 1800 create matched pairs.
Commonly...
24 x 75 =1800 so 1/24 = 75 mm
20 x 90 = 1800 so 1/20 = 90 mm
15 x 120 = 1800 so 1/15 = 120 mm
12 x 150 = 1800 so 1/12 = 150 mm
10 x 180 = 1800 so 1/10 = 180 mm
9 x 200 = 1800 so 1/9 = 200 mm
8 x 225 = 1800 so 1/8 = 225 mm
6 x 300 = 1800 so 1/6 = 300 mm
4 x 450 = 1800 so 1/4 = 450 mm
Please note that is SCALE, not SIZE.
If I have this wrong, please advise.
Numbers go in my head and they get stuck there......