Minimum tube diameter?

I'm designing an airline-carryon 8" f/6, and I'm trying to pin down some of my dimensions.

How do I go about calculating the minimum inner diameter for my upper cage? I've found references claiming it's (diameter of primary) + (field diameter). Is this correct?

How do I calculate the field diameter?

-David

Download "Newt.exe". It is easy to use and will give you all the dimensions you need to make good design decisions.

http://home.att.net/~dale.keller/atm/newtonians/newtsoft/newtsoft.htm

Lee