Confused by Newt re focal length and mirror positioning

I've been playing with the Newt, and having built my box up fron
before I got my mirrors, the only parameters I can really play with in
Newt are the focuser to the front of tube distance and mirror to the
back of the tube distance.

Now in reality I'm assuming that the two are intrinsically linked, in
that the distance of the light path from front of the primary mirror
along the tube , bouncing off the secondary and up to the eyepiece
should equal to the focal length (I know the exact equation is a bit
more complex than this, but I'm simplyfying for brevity).

Now then, I have previously sat down and calculated where to place my
primary and secondary, based on the focal length of my mirror.

My question about Newt is as follows:

I assumed that the positioning of my secondar and primary mirros were
crucial and heavily realted to the focal length of the primary mirror.

Why is it then that Newt lets me move my primary back and forth
without telling me that the focal point is moving somewhere stupid ?

I would have thought for instance, that moving the primary halfway up
my tube would have pushed the focal point about a foot above the top
of the eyepiece, but no, according to Newt it's always in the
eyepiece.

What am I missing here ?