Forget millimeters, if God had wanted us to use the metric system there would have been 10 disciples...lol
seriously though,..
"Dial calipers,or micrometers,for accuracy.
Most of the marble people I know are happy if your within 1/16th,of an inch.With the exceptions being pee-wee,1/2( 0.50),or any other bench markes size.You know the ones,,over an inch,,,over 3/4,,,,over 2 inches,,,ect,,ect,,,,'
^what he said^..
and what Steph said about the metal calipers. I think you can pick up a plastic caliper for less than the price of a couple gallons of gas if you shop around the internet.