The below image (and attached documents) shows where to float the sleeve weight to with a 2465 piston gauge.
This mark is known as the reference plane and is used by both WinPrompt and the COMPASS for Pressure Piston Gauge calculator tool to calculate the fluid head pressure for a Device Under Test. In practice, when measuring for the DUT to Reference standard fluid head height, the measurement is made relative to the 2465's reference plane.
