So I was trying to make a mount for my new Sony cam which has pre-drilled M2 (2mm) mounting holes. Just the wrong size for a 4-40 standoff!!!
Here's the formula for Unified threads -- those are the ones with a #1 - #10 at the front.
major thread diameter = 0.060'' + (0.013" * N)
The second number (i.e. 40) is the number of threads/inch.
So a #4-40 has a major diameter of 0.112", or 2.8448mm.