Okay, thanks for that clarification (likewise to others who've chipped
in, too).
I'm building up a picture, but it's not so easy in this text-only
medium.
Okay, so I think it's overwhelmingly clear that adding or subtracting
any electrical length from this "baseline" - (or "reference plain" as I
think it's also called) introduces measurement error which must be
allowed for/accounted for in some way to maintain accuracy.

Let's say I make up a short and an open using N-type plugs as described
elsewhere on this thread, using identical, high quality plugs. Can I
then use these with my proprietory Suhner 50 Ohm precision N-type load
by calibrating out any differences between the home-made jobs and the
proprietory one?
Further, I now know that a "through" is simply that U-shaped patch lead
that's often seen sticking out of the front of VNAs in catalogue
pictures. I also know that you can't make one of these with zero coax
length for obvious reasons. Given the fact then that it has an
unavoidable length; does it have to be any *specific* length in
relation to the open, short and 50 ohm terminations?
Thanks.