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"Typical" seems to be 55mm from the base of the camera side threads so that is probably a good start. You can use programs like ASTAP to examine field curvature after that which will give you average star HFR in the center and the corners, take a couple of shots and average the readings, add or subtract a mm or so from the spacing and take some more shots to see if the corners got closer to the center HFR or got worse, to see if you are going the right way. ASTAP will also give you a graphical representation of tilt. And it is free!
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