This is a common wish. We want to be able to DISPLAY calculated values not only in schedules but also in tags. Right now lacking this feature makes me feel like this guy!
Thanks to this site for the image.
Amended 09/11/08 - Per the comment, note that I'm asking to DISPLAY a calculated value in a tag. I'm not telling anyone how to accomplish it. I'm asking to be able to use a tag family to report a value that is the result of a calculation in a project relative to any element.