mx5cartalk.com

Haven't done this myself, but hope the following helps:

Keith Tanner in Mazda Miata MX5 Performance Projects says "between 0.010 inch & 0.020 inch (0.25mm to 0.5mm) is a good range".

Norm Garrett in Mazda Miata Performance Handbook says "every 0.065 inch you mill off the head removes about 0.15 cubic inches from the chamber & increases the compression ratio approximately half a compression-ratio point".

the best way to increase compression is via hi-comp pistons. decking or shaving the head, will only net you a very small gain in compression.
same situation if you were to fit a thinner head gasket.

my b/f adam used wiseco hi comp pistons in his ae86 4age engine and went from 10.5:1 factory, to 12:1 using pistons.