What's the Gauge of a Woodscrew?

And a scheme for working out the clearance hole

630Measure the head in sixteenths of an inch, subtract 1 from that answer and then multiply by 2. So far so good, but a reader extended this by showing how to calculate the clearance hole for woodscrews as the gauge plus 3 in sixty-fourths. ( For example 10 gauge: 13/64").