It might be me but that website description makes very little sense to me. Just look at the relationship between H1 and h3.
I also tried to calculate to the outer point (above D) and that made no sense either.
If you use a thread pitch of .75mm then the long side of the 30 degree triangle is .65024 mm again that does not correlate with H1 or h3 or even (D-d3)/2 which should be very close.
I am not sure this diagram is correct at all.
Dan