Where on the threads would I need to measure to find the nominal size
of a bolt?

If I measure the major diameter of an external thread (bolt), the size
is always considerably smaller than the nominal size. I.e. if I
measure an M12 bolt, it will measure around 11.8 mm or so. Similarly,
if I measure a 1/2-inch bolt, it will measure around 12.5 or 12.6 mm,
where 12.700 would be exactly half an inch.

The problem occurs when I want to make an external thread on the
lathe. So far, I have always turned the stock to the nominal size (12
mm for an M12 thread), and then cut the treads. This consistently
produces terrible results, where the nut will not fit until the thread
is cut way too deep.

So, I suspected that maybe the nominal thread diameter would be
referenced from the theoretical point where the flanks of the treads
would meet to a sharp point. To check this, I drew a quick thread in
Fusion 360 (it has built-in threads), and did some measurements there.
Alas, it still does not work out. In this case, the size comes out too
large, by about the same amount it was small before.
--
RoRo