WARNING: This page is about a different car, the 2010 Audi A5, 2009 Audi A5, and 2008 Audi A5. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: Cylinder Head Refacing Dimension
Courtesy of AUDI OF AMERICA, LLC
Resurfacing cylinder head (face grinding) is only permissible to minimum dimension -a-.