.1 mm (one tenth of a mm) = .004 inch which is the specified valve clearance.
Put the "T" in the window, centered as best you can. This is only to check to see if the cam is in the right position.
The cam is in the right position when two circles or holes in the cam sprocket are lined up parallel to the flat surface of the head (where the cam cover seals) AND there is a circle or hole in the cam sprocket visible near the edge like the other two holes or circles. There should be a little "play" in both rocker arms and this is the clearance that should be .004" with a feeler gage.