Eh, ended up doing this as an activity to satisfy myself. I've created a spreadsheet which will calculate separations and off-sets for a Newtonian. Simply whack in Primary and secondary diameters and an F ratio, and Bobs your cross dressin aunty... so to speak. I'm pretty sure it will work for any Newtonian configuration. I put in specs of the scope I am building and it correlates with my CAD design. (No warranty given or implied, primarily due to the empty shot glass on the desk next to me...
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Perdy self explanatory...