I’ve been using Illustrator for years, but today I discovered a new trick. (Well, new to me, but probably common knowledge to everyone else.) You can enter simple math into the Transform dialog, in any units, and Illustrator will handle the conversion for you.
It will handle addition, substraction, multiplication and division, but not parentheses, and not multiple operations at once. But for quick-and-easy adjustments (e.g., kerf correction), it’s a handy feature.