This engine should be shimmed for 30% straight from the factory. I run my Mugen X!2 with 30% on stock shims (0.022" of clearance).
To check clearance, take out the glow plug, put in a small bit solder lead so that it goes between the plug and piston, rotate the engine one turn, pull out the solder lead, and finally measure wth pinched portion of the solder with a digital caliper.