My son spent another week in Tebis training last week...
http://en.wikipedia.org/wiki/Tebis

So, I asked him what he learned. He's now so far into tooling design I
can hardly follow it. he's now lead CAM programmer for his company.

But he did tell me this little fact. For a tap drill size you subract
the recipricle of the lead from the nominal size. ie. for a 3/8 x 16
thread subrtract 1/16 from 3/8 and use tap drill size 5/16.

For metric taps subtract the lead. ie. for M10 x 2.0, 10 -2 = 8 mm tap
drill size.

I had always just looked at my handy tap chart.