The instructions for calibrating zoom in the MAC TW1 user manual Revision A contain a minor error. The correct procedure is as follows:

To calibrate zoom, first remove the bottom head cover. In the onboard control panel, select UTIL-->CAL-->Z OF and press [Enter]. Run the zoom car to the rearmost limit of its travel (i.e. furthest back towards the lamp) where it is pressing both end stop switches fully closed. Adjust the zoom car forwards slightly until you hear a 'click' from the end stop switches as they open. Press [Enter] to save the setting. Replace the bottom head cover.