What sort of camera? What brand of flattener? What sort of scope?
On my rig it's 55mm from back of flattener to the sensor of the camera, too many variables
If it's the 2.5 inch flattener then this will help
Refractors with a focal length up to 500 mm 106 mm distance Refractors with a focal length between 501 mm and 600 mm
101 mm distance Refractors with a focal length between 601 mm and 700 mm
96 mm distance Refractors with a focal length between 701 mm and 800 mm
91 mm distance
Refractors with a focal length between 801 mm and 950 mm
88 mm distance Refractors with a focal length above 1000 mm
87 mm distance