Expanding-universe

Hi All: my scope achieves focus when the drawtube is all the way in. Unfortunately I need more space for the ZWO EAF to do a focus routine as it needs to go past the point of focus to create the V curve. I do have the required 55mm of back focus consisting of the following: zwo cooled camera, 11mm spacer, ZWO filter drawer, ZWO OAG and the William optics 6Aii flattner set at 7.9mm. My scope is a GT81 from William Optics. my questions are this: do I need to add or subtract some space between the camera and the flatner? If yes, how much and where in the image train should I add or subtract? many thanks, in advance.