Thanks for the replies everyone.
Ok dusted off my trig, results in the chart attached. My widest eyepiece has a field stop of 38.7, so I am using a field size of 39mm. The light cone is 39mm across at 585mm from the focal plane.
http://i305.photobucket.com/albums/n...1.jpg~original
Feel free to check or correct, I haven't done trig since I left school 20 years ago. Assuming I have the maths correct, I can use the 'berfield' method for placing the baffles?
Regards
David
PS excuse the crappy diagram, I now know why paint come for free with windows!!!!!