The big plus of the Newt software already mentioned is that it will highlight any design flaws in the parameters you have chosen.
This site
http://www.atmsite.org/contrib/Fly/s...designie5.html gives you control over any variable you care to mention and can calculate a lot of unknowns for you, provided you know some simple facts about focal length etc.
I suppose it could also be used 'in reverse' to design a scope that has traits that you want, as opposed to newt which will tell you if what you have is going to function properly.
Also contains secondary offset and eyepiece calcs.
Hope this is of some use