Maybe too much skimmed off

Hi everybody, hope someone can throw some light on this problem.
I have a 1997 disco 300 TDI with head gasket blown. I have had the head skimmed but they have had to take off 18thou to get it flat. Now I am concerned that the valves will clout the pistons. Does anyone know how much clearance there is between valve and piston or is there any way of knowing whether they will touch. When I removed the head it had the thickest gasket already fitted but that could have been someone just being on the safe side. Thanks in advance, Brinks

In addition to this problem how much can be skimmed of the valve seats. Thanks, Brinks.