Recommended practice, for use in conjunction with API Chapter 2.7 Calibration of Tanks on Barges, and API Chapter 2.8A Calibration of Tanks on Ships and Oceangoing Barges. Establishes reference gauge heights during calibration of marine tank vessels. A reference gauge point is necessary for converting ullage to innage and when determining the volume of the quantities remaining on board ROB. A reference gauge point is also used for wedge formulas and establishing wedge tables.