A couple of gaskets I have here to measure, a new factory Hayabusa MLS is 1mm over the bore size.
A factory Mitsubishi Evo MLS is 1.2mm over the bore size, Mitsu does not offer oversize head gaskets so I expect this has room for .5mm overbore piston sizes.
I would presume that the 1mm over size John recommends is the proper thing for us to do.
'66 Hustler cafe build
'78 GS 750E sitting
'79 GS 550 ex racebike soon to be a 740
'83 GS 550ES with a 5-11 swap
'75 Ducati Sport, stock and will remain so