ATM Re: diagonal to eyepiece distance

[Carl Woebcke <carlw@telis.org>]

Mel and ATMs,

I'm stumped.  You've told me that "the minimum diagonal size is the
diagonal to eyepiece distance divided by the focal ratio".  Assuming
that its a virtue to reduce CONTRAST, I'm trying to determine the
minimum focal ratio on an 18" Dobs (to be built) that allows a 2.61"
diagonal (giving me 14.5% obstruction as opposed to 17.2% with a 3.1"
diagonal).  Therefore I need to know how to determine the minimum
diagonal to eyepiece distance (using a JMI NGF-2 focuser with a 1.53"
min. height), which is alluding me.  I'd like to mount the focuser with
the TOP of its base flush against the INNER face of the board that holds
it, so that the BOTTOM of its base is as close to the light cone as
possible, minimizing diagonal to eyepiece distance, diagonal size, and
focal ratio/tube length.  To do this it seems that I need to be able to
determine 2 distances: 1) HOW FAR UP FROM THE BASE OF THIS OR ANY
FOCUSER DO YOU ASSUME THE FOCAL PLANE TO BE?, and 2) how close to the
light path of an 18"er can I place the base of this (or any) focuser? 
Is my thinking on this correct, and what am I missing?
Thanks in advance.

Carl Woebcke