Shim count is not as important as "actual deck height" . Take a piece of small soft solder and bend the end so it will go into the glow plug hole then rotate the piston past TDC with the solder facing the carb. You should have at least .006" per 10% of nitro as a rule of thumb when measured with a caliper or micrometer.
Ex.: .006" X 3 (30% nitro)= .018" min. ( .021" is better, especially in hot climates)
Hope this helps!