ok, i have brand new standard spec cast mahle pistons ready for my engine build

im looking into lowering the compression slightly, and was wondering how much is to be taken off the face of the piston to achieve this?

if i get this done via a specialist tuners, they merely send the pistons to a machinist and charge me extra for the priviledge, whereas if i know the amount to be removed, i can go direct to the machinist and get it done at half the bloody price!! lol

so, anybody know the typical amount removed for a typical lowered compression ratio on a s2?