Detonation. More than .2mm needs to be shaved

[spooledrotten]

just bought an s14 silvia and the previous owner just replaced the head gasket. Well after fixing a boost leak and hitting 1.1 bar I blow the new OEM gasket. After taking the head off I see that it failed in the same place he said it did before, cylinder 4.
I drop the head off to get machined and after .2mm has been taken off (the FSM stated maximum) they tell me they'll need to take off another 0.3048 to make it level because of the detonation around cylinder 4.

MY QUESTION IS.
Although I'm past the .2mm that the FSM allows can I just compensate this by using a thicker head gasket and nothing else?

I checked the block with a straight level and a flashlight and it seems to be bang on so I'm not too worried about that.

I've never taken an engine apart. I thought this was gonna be an easy take off, machine, MHG, BOOST. The stock thickness is 1.1mm so if they take .5048 off can I use a 1.6mm? Or would the .0048mm difference push me more towards a 1.7? IF this is even possible to solve my decking problem with a HG.
THANKS