Hi Peter
You shouldn't need a spacer. The quoted back focus of 56mm with the flattener is the distance between the flattener and the focal plane of the camera, ie, the point at which an object is brought to focus.
I believe that's 44mm for a Canon d60a DSLR. (See link
https://en.m.wikipedia.org/wiki/Flange_focal_distance) then add on the thickness of the OAG which I think is 10.5 mm per this link
http://www.bintel.com.au/Astrophotog...oductview.aspx.
and you are at 55.5mm. In theory close enough not to need a spacer but annoyingly not enough to give me pin point stars at the corners. Close, but...