I know it’s picky, but while I applaud the general advice, what is typed here does not work. It seems that Rhino expects inches to have a value less than 12 and eighths to have a value less than 8 within feet and inches notation…

I believe that the OP is requesting that these rational limitations be lifted in order to simplify his mental arithmetic, as in 5/8 + 1/2 = 9/8 (and let Rhino convert that to 1-1/8).

Of course, he could also adopt the metric system which does not have these pesky fractions.

Or make the command line arithmetic feature work with feet and inches notation, like it does with metric values, so you could type 3’4-7/8"+45’9-3/4" into the command line. Or maybe (3’4-7/8")+(45’9-3/4") to simplify the parsing.