WARNING: This page is about a different car, the 2012 Audi S5, 2011 Audi S5, 2010 Audi S5, 2009 Audi S5, and 2008 Audi S5. 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-.