ATM Diagonal Question

[machead@qadas.com (Bob Hazel)]

 My scope is a 8" f/6.7. Tube is 10" ID and about 1/8" thick. I am planing
on a 2" height focuser racked all the way in. This scope is not being used
for photography at this time. Came up with 7 5/8" (added an extra 1/2" past
racked in focuser). I plugged this into Newt20 and used a 1.53 minor axis
diagonal. The specs looked pretty good to me. They also looked ok to me
when I plugged in a 1.3 minor axis diagonal. So my question is, what size
of diagonal would give me all that the 8" mirror has to offer?

thanks

bob, Denver, CO