Mick I had similar issues in designing a mounting block to fit a 2 inch focuser to my little vixen newt and needed to know focus heights of various eyepieces. The good folk at CN were able to help me out with these links.
http://www.jim-easterbrook.me.uk/astro/ep/index.html
http://www.televue.com/engine/page.a...nt=True&ID=144