this is essentially as mathematics problem than a CAD one.
i mean formula for height of the wet dip stick or height in manometer tube should give volume of liquid present in the cylinder.
cylinder is horizontal on the ground.
If you search the terms "volume of a horizontal cylinder" you can get quite a few explanations.
This is one example;
You would need to solve for "h" or height for the volume you wish to indicate on your gauge, then mark the gauge at that height.