This is what the camera actually sees: When the turntable is rotated the "distant" marker will move sideways relative to the "near" marker, either one way or the other. This is checked with the camera at varying positions along the marked line. Eventually one single position will be found that results in NO visible movement of the two markers - they will remain exactly centered one above the other no matter which way and how far the turntable is rotated. The entrance pupil will now be located exactly above the turntable's pivot.
At this instant the distance that can be read off the scale from the turntable pivot to the center of the ball head carrying the camera will represent the actual distance between the camera's tripod socket and the entrance pupil of the lens at the current zoom setting. This is the dimension that has to be marked out when manufacturing the pano bracket.