I guess the trouble with modern optics, especially DSLR lenses actually, is that they consist of multiple elements and it's not always obvious where the center of an equivalent single lens would be. But actually that doesn't matter because all lenses, telescopes included, have a headline focal length.
And I'm not just theorizing here as I'm definitely putting my money where my keyboard is. I have a new 'scope on order and it has a focal length of 980mm. A ray of light passing through the center of the aperture can be pretty much assumed to pass through undeflected (same deal with a reflector apart from the fact that the ray doubles back on itself and angle of incidence equals angle of reflection) so I've used that together with tangents to calculate the field of view from the center of the sensor out to the sides and then doubled that figure to get the total field of view. Those calculations did prove expensive, however, as I rapidly convinced myself I needed to specify a CCD camera with a bigger sensor!
Usable FOV might be another matter, though. Newtonians can have a limited FOV over which everything is really sharp. The telescope's documentation should help determine if that's a factor for you.