It would be awesome to have a built in calculator the can figure out various calculations such as diameters, circumferences, scale conversions, etc. Scale conversions always throw me off. If I have a file that is made at a 1/20 scale and I want it at 100% I would love to be able to just plug that in.
You can already enter equations into the width, height, position, and scale fields, and they accept units too. For your “1/20” case, you could just add * 20 to the end of the “100” in the percentage size box, and that would work for you. It will also do unit conversions, and I’ve added " for inches and ’ for feet in the next release.
In addition to this, the rotary setup dialog auto-converts between circumference and diameter, so the ones you’ve listed so far are already supported.