A lot can be said about this interface.
An equal sign between two clearly non-equal values is some American nonsense scale notation that abuses the equal sign. This simply isn’t what the equal sign means. 1 mm can never be equal to 100 mm (unless mm would be defined as 0, which it isn’t). Also all the extra noise makes it hard to find and read the ratio that is the actual scale. I’d very much prefer if the scale were only labeled “1:100”, 1:200" so you easily can pick the right scale.
Typically when I install SketchUp on a new machine I delete all shipped scales and start fresh to get rid of the noise. Adding custom scales is a much more cumbersome process than it could have been as LayOut by itself adds a lot of irrelevant noise to the name field, that has to be manually erased for an easy to read scale.
The unit dropdown doesn’t make much sense either (it doesn’t even default to document units!). 1m:100m is the same as 1mm:100mm or 1 banana:100 bananas. A value divided by itself is 1 and therefore has no reason to exist in both the nominator and denominator of a division, which is what a ratio is.
For the rare situations when you want different units in paper size and model size, LayOut could simply parse the unit from the entered value as SketchUp and Layout does on all length input. No extra compulsory field is needed for this.
Another approach would be to not have predefined scales at all, but instead allow the user to write “1:20” or whatever the scale we want, when we want it. Just writing it is faster and simpler than selecting it from a list. Especially when the list is bloated by noise, or misses the scale you want.
It is very simple froma programming point of view to split the text on “:” (and “=” to also allow the nonsense notation), parse the two texts as lengths with SketchUp’s default length parser and divide one with the other to get the scale factor. Actually quite a lot simpler than the current implementation.