Hi Andy
Have a look at this site.
http://www.howardedin.com/articles/fov.html
Select your camera (40D) and enter your lens focal length and it provides the FOV.
For your telescope, the focal length will be 770 x 0.85 = 655.
Another very useful resource is:
http://www.wilmslowastro.com/software/formulae.htm
Cheers
Dennis