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like this?
on mac, you can set up text replacements so anytime you type 5/8, it shows up as (hmm- it won’t work in this particular reply field… but it’s the unicode vulgar fraction)…
they’re sort of limited regarding typical architectural dimensions so if it were to be implemented in rhino, i’d imagine it would require a custom font to be made… at least going all the way through 16ths… but if the effort were to be made to do such a thing, it should probably go through 128ths…
as of now (again, on mac at least) this is what fractions replacements are limited to…
…of which only 7 of them would actually be of use in architectural dimensioning