Place a target on the ceiling.
Put one tribrach on a tripod and a tribrach adjuster on that tribrach.
Turn the tribrach to be adjusted upside down so that it faces the target. Then, attach this tribrach to the adjuster.
Adjust the leveling screws of the bottom tribrach until the crosshair of the top tribrach aligns with the target.
To check if the crosshair remains aligned with the target, turn the top tribrach 180 degrees on the adjuster.