Originally Posted by what_stanger?

Add 0.1mm shim to the engine.

Just make sure you know, shims aren't all 0.1mm.
Stock the engine should have two shims;

1) 0.2mm
1) 0.1mm

that gets you a total of 0.3, you need one more 0.1 for 30 percent(0.4mm). You could also use two 0.2mm shims if you wanted, to only have to bother with a total of 2 shims.

EDIT: if you want to run 20 percent, use 0.3mm in shims total (probably the two the engine came with....although i'd still check)
if you want to run 30 percent, use 0.4mm in shims total