Threading and the compound angle.

[SmithDoor]

From Machinery's Handbook, table of dimensions for National Unified Threads
10-TPI, pitch 0.100", depth of external thread Sharp Vee 0.0866,
depth of external thread National Unified 0.06134

so the pitch is 0.1000", if your cross slide is at 30-deg and you feed in 0.075"
on the cross slide you'll get a final depth of 0.065", not equal to either the
Sharp-Vee or the National-Unified depth (and 29-degree goes even deeper
and more off)

so I'm confused, it still looks to me like you're using a "rule-of-thumb" that is
close but not exact ? and on top of that, all of this depends on the shape of
the tip of your thread cutting tool, you didn't mention that in your original post,
what shape tip are you using ?, what shape tool are you recommending ?
Curious minds want to know !

There are different angles you turn compound to besides angles for threading.
Back before the 1960's some machinist would set the compound to 30° so feed would match the mic or on diameter.
This done for tool post grinding to for finder feed angles 11° down to 3°.
You also do metric or inch feed at correct angle.

Dave