I went about it in an awkward way.
I look at the image on screen (even though it is rectangular) and then go into Cartes du Ciel and place the EP circle to cover the same area as the camera sees (the FOV on screen). By gradually adjusting the circle size by the numerical settings it tells me what size the image circle is.
Awkward and fiddly but it works
Here is a screenshot showing what I mean. All using an ED80 and M42 as the guide.
*I have the inner circle showing me my FOV in the Toucam.
*The next circle shows me the FOV of the Toucam and a 0.6x Focal Reducer
*The outer circle shows me what my finderscope sees.
On the left is the circle sizes of each.