A level gauge system for mounting on a roof of a semi-closed storage tank includes an automatic tank level gauge for determining a level reading for a liquid including at least one liquid component in the tank. A processor includes a memory storing pressure correction factors or a pressure correction factor equation for correcting the level reading for a measured gas pressure above the liquid in the tank. The processor is programmed for implementing choosing a selected pressure correction factor from the pressure correction factors or the equation based on a received current gas pressure above, and applying the selected pressure correction factor for automatically correcting the level reading provided by the tank level gauge to generate a corrected level reading which compensates for the current gas pressure above effects on the roof and on the level gauge.