Chuck that 1mm gasket and get a .2mm gasket.
Oh boy, I'm gonna start some controversy here
The number one thing to remember is trapped volume vs compression ratio.
In other words, how much air by volume are you compressing and how many times are you compressing it.
All other head design aspects are important but secondary to trapped volume vs compression ratio.
At sea level atmospheric pressure is 14.7 lb per sq in
At 5000 ft it drops to 12.2 lb per sq in
So lets say that you are compressing X volume at 13.7:1 compression ratio
At sea level that's 13.7 x 14.7 = 201.39 psi cylinder pressure.
At 5000 ft its 13.7 x 12.2 = 167.14 psi cylinder pressure.
The higher in altitude you go the lower your cylinder pressure is.
On a KX500 for good all around performance you want your cylinder pressure to be around 170-200 psi.
Unless your head, cylinder, or cases have been machined (and by your measurements they likely have not been) using the .2mm gasket is your wisest choice.
Otherwise you are lowering your compression over stock numbers as you are adding approx 1.5cc volume per .2mm gasket thickness... In other words you have raised the volume by a total of about 4.5cc greatly lowering your compression ratio. Then factor in the altitude and your gonna have a real lemon using a 1mm gasket...