Dimensioning Pipe?

I recently completed drawings for a item that included some steel pipe
for a pneumatic line. [About 30' of pipe, with about 25 fittings] I
obtained dimensions for each of the fittings we needed, and
dimensioned the length of pipe by indicating the distance between
fittings. I ignored thread engagement, thinking that this is a
variable I couldn't control. I was also thinking that the installation
must, by nature, be custom fitted by the guy installing the pipe. One
experienced person said that thread can be pretty variable, and the
length of thread engagement will vary depending on how deep the maker
cut the threads, and how worn the tool was.
At some point in the past, I saw a piping drawing that gave
distances to intersections of pipe, ignoring the fittings. But, this
doesn't seem any better than the method I used, except that it doesn't
require me to know the real size of the fittings.
The item I drew is being built now, with the assumption that my
dimensions are only a general guide, so each pipe is being cut to fit
after prior pieces are already installed. I am evaluating if there
might be a better way, especially in situations where I have a number
of fittings close together and want to use standard size nipples.
Perhaps I should assume a certain depth of thread engagement and give
the total length of the pipe (including the thread)?
Any suggestions or industry standards?
Joe Dunfee