Don't just look at the shims and assume it's your head clearance, it's not in most cases. Take off the button, turn the piston into uppermost position (make sure the liner does not lift) and measure the distance between piston and upper edge of the liner. This will be your measurement A.
Then remove all shims from the head button and measure the height of the button part that goes into the liner. This is your measurement B.
Now do A-B, this equals your head clearance with no shims. Can be both positive and negative (although I've only seen positive, but I haven't seen too many of them
). If the result is, lets say 0.3 mm, this means you should only use a 0.1mm shim to get a 0.4mm head clearance. Around 0.4-0.45 is what I've often seen suggested for 0.12 engines and 16% fuel.
In real life things can be a bit more complicated, if the piston has a dome or cavity, the button is not cut straight, but at an angle, different volume of the button cavity etc... they all affect compression. So it's not 0.4mm set in stone, but depends on the engine, application and environment.