I have the same type of setup. An RC 8" Astrograph with a 0.75x reducer.
To figure out the correct camera use this formula. ((pixel size of camera sensor divided by focal length of OTA) x 260). Use the result and use the table in the resource section of Atik website. You should be able to select a camera that is close to a 1:0 ratio. It reduces the likelihood of over saturating or under-saturating stars.