The thickness of the head gakst has NOTHING to do with how much you have skimmed, save perhaps for what Burgerman mentions, the valves being closer to the piston at TDC.
Anyone rebuilding the head should check the valve clearences anyway, so that should be taken care of (they will all be too tight if the head is skimmed).
I have a LR original publication manual that states the 300 TDi head must NOT be skimmed but renewed, though many do it with no problems.
The deciding factor in head gasket thicknes if what's called piston projection, (HairyAssWelder found the page somewhere), to obtain the correct compression ratio it's important that the piston almost touch the head, save for a coat of soot, so you measure the height the pistons stick out fo the top of the block at TDC and pick a gasket that leaves them the correct distance below the head/gasket surface. Obviously you have to compramise if the readings vary.
Stick too thick a gasket on and you get low compression, with all the problems that entails. I've heard stories about LR technicians being told to just stick the 3 hole gasket on "to be safe", well that's just
lazy. The procedure to do it right is written down somewhere so these "technicians" can do the job properly.
However, if you do not disturb the bottom end at all then you can just stick the same gasket on as was already there, but be careful, there are some gaskets knocking about with the wrong number of holes in them due to a "tooling error" (more likley an erro on the part of the tool making them
) and if some lazy are in a LR dealership has been there first, you will only replicate their mistake.
To answer the original question, obvioulsy the minimum possible but as LR do not advie skimming the head they provide no technical data to do so.
If the skim is substantial then you could re-cut the valve seats to move the valve heads back up away from the piston, but I took up to 1mm off ours with no problems. I say up to as the head was bowed, concave, so more came of the ends.
