Just be aware that if you try to use Imperial/US Customary units in the measurement box in a drawing set to metric units, you get very confused results even in the latest version of SU, especially with fractional inches.
See SU doesn't recognise fractional inches (e.g., 1/2") as an imperial measurement when units are mm
There are several past Feature Requests asking for the measurement box to accept arithmetic expressions. In general, it doesn’t. There are a few exceptions, which I can’t remember at the moment.