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Interesting scaling, in the UK 20mm is normally nearer 1/76 (or 4mm to the foot or UK 00 gauge scale *).PSC label theirs at 1/72.US and Continental HO (1/87 or 3.5mm to the foot) sounds is close to the quoted 1/90.Back to the top of the head or eye level... (* it is OO gauge because it uses HO gauge track at 4mm to the foot scale, but that is a story for another day)
Yes, I've never heard of 20mm = 1/90. 1/76 pretty much and very generally applied to 72 and 76, though in the last century, the former was frequently referred to as 25mm.