We have had this functionality since pretty much the beginning. Just type 30-2 and press enter. Also, if you are in inches, you can type in 32mm and press enter and it will convert it for you.
Thanks Scott, I've used the mm conversion all along. So I've never tried to add and subtract by entering the original number again. Seems a little redundant but works either way.
As a test I opened a cabinet in CE and tried to +.75 but got a syntax error. So I removed the " symbol and placed the +.75 and it worked. Wish I thought of that a long time ago.