Ned Simmons writes:

You should have a flat equal to 1/8 the pitch at the major diameter of
the thread. The thread crests should be truncated to about 1/4 pitch
at the minor diameter.

Which just begs the question: what are the major and minor diameters?

The ideal thread form has no direct application in practical thread
cutting. It's kind of like saying holes should be round. It doesn't tell
you how out-of-round your real holes can be, or how much over- or under-
sized they can be. It is the latter allowances that determine how to drill
or bore a real hole.

To cut real standard threads, you must stay within allowances that are
rather randomly related to the ideal forms in both sign and magnitude, and
which do not necessarily even include the ideal dimensions.

While the tables in Machinery's Handbook are correct, they only tell you
the range of allowed results, and not the practical tool shapes and cuts
that yield those results.