stock squish is around .024" (.6mm), ....017" is considered the minimum by zenoah ....stock blue paper gaskets are .020 and compress to probably .015...if you are using gasket seal without paper it'll compress down to around .005-.003" which will give you a net of around .0014". some boat guys are running .013 as an absolute minimum so i'd say you're close to that...you get into pre-ignition problems rather than striking the piston on the head...engines do not like pre-ignition. i tried running .013" before formore compression and motor was hard to start and ran hot so i aborted the idea
if you say you have .5mm (.019") then you should be looking good but make sure your solder is hitting the cylinder wall before you squash it...that's where you take the squish reading.
and i'd recommend using a base gasket rather than just goop...a properly torqued head wont leakat the base gasket...torque the bolts to 59-63inlb's and then retorque them after a heat cycle on 2 bolts, 4 bolts are usually good to go after the initial tightening
also when you rebuild a motor and use oil in the cylinder and on the ring during assembly it'll have noticeably more compression...once it fires up though and the thick oil burns off the compression will drop back to where it was before