Lengths and Distances can be entered in a variety of formats;

12ft 4 1/2in, 12' 4.5", 148.500, 1.234m, 123.4cm or 1234mm are all valid values.
Adding the Equal Sign (=) key after an entry will reformat the value entered to your preferred length format (Preferences Dialog).
Adding the "`p`" key after an entry will convert a prototype measurement to a scale measurement by dividing the value entered by the ratio for the current scale.
Adding the "`s`" key after an entry will convert a scale measurement to a prototype measurement.

The conversion will happen when the Tab key is used, or the cursor is moved to a different entry box.