File at YouTube: https://youtu.be/fuCJ52hSta4
This portion of the treatise is not a test procedure. It explains the static facts concerning the relative positions of the various components in the spoolbox.
Since the position of the take-up is set at the factory and almost never changes during the life of the mechanism, it is the best primary 'reference point' for adjusting the tracking mechanism. With that in mind, when everything is correctly adjusted, the edges of the music roll paper will be an equal distance from the end flanges of the take-up spool. Next, when the trackerbar is correctly orientated with the take-up spool, I note middle "C" on the Test Roll will correspond with note middle "C" on the keyboard. (This is mentioned here because with a transposing trackerbar there is an adjustment where the linkage from the transposing lever attaches to the trackerbar which allows you to adjust the horizontal position of the trackerbar.) Also, it's important to mention that in the "O" (zero) position, the transposing lever is in the center, or #3, slot (of the five slots). Last, we come to the horizontal position of the music roll. As you have no doubt noted, the position of the roll is controlled (or determined) by the lever to which the string (from the tracking mechanism) is connected. And, attached to that lever is a piece of metal that pushes on the right roll chuck. You'll also notice that there is a set screw which secures that piece of metal to the lever arm. That set screw is adjusted at the factory and should never be changed. (It is one of the calibation points of the tracking mechanism.)