Identify Vertical Sill Mark Shown by Label and Notch in Rail.
Check the level of the sill to determine the lowest side
and mark it for future reference.
Place the sill mark/notch on the rail even with lowest
side of the sill (if the sill is not perfectly level). The
lowest side establishes the trucking sill position for both
sides of the door trucking sill. Make sure both sides of the
door sill are even with, or just below, the landing sill.