I know this is a very old thread but now that it's been resurrected ...

The simplest way to check letter gauge is to buy a cheapo plastic vernier caliper from Bunnings or similar.

They only cost a couple of dollars and will accurately measure the thickness of your item.

The envelope only adds half a millimetre to that figure.

This tool is also useful around the house for measuring thicknesses, lengths and internal widths of holes etc