When entering values into dialog boxes in 3D Studio (Autodesk VIZ, 3ds max), if you enter the value with the letter "r" preceeding the number, that value will get interpreted as being a relative amount.
For example: Suppose that you previously extruded a wall 8'. Now the design requirement is that the wall must be 8'6". Rather than having to type 8'6" in the dialog, simply type "r6"" (without the outer quotes) and the wall will now be the desired height.
Note that it does not matter whether the letter "r" is lower or upper-case.