The best is the following:
Put piston at TDC, and then unscrew the cooling head and headbutton.
Measure the distance from top of liner to top of piston.
Also measure the distance the headbutton goes into the liner.
The difference (head clearance) should be around 0.2mm, but I have seen headbuttons where this distance is 0mm.

Now shim the headbutton, until the head clearance is 0.5 - 0.55mm for 25% Nitro, at sealevel...