Maximum permissible refacing dimension, calculating

WARNING: This page does not describe the selected car, but rather 7 other vehicles, including the 2004 Audi A6, 2003 Audi S6, 2003 Audi A6, 2002 Audi S6, and 2002 Audi A6. However, it is still accessible from the selected car via links, so may be relevant.

Install valve and press it firmly onto the valve seat.
NOTE: If valve is to be replaced, use new valve for measurement.
Install valve and press firmly against valve seat.
Measure distance between end of valve stem and top surface of cylinder head.
Calculate maximum permissible refacing dimension from measured distance and minimum dimension.