Calculation of the liquid level in pressure storage tanks

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If a pressure tank is filled with liquefiable gas and a two-phase equilibrium between gas and liquid exists inside the tank, how can the liquid level in the tank be determined given the temperature and the amount of gas filled? Background of the problem: Our pressure storage tank is equipped with a differential pressure level transmitter, and we hope to correct the readings of this transmitter through theoretical calculations. Approach: Personally, I think it can be calculated this way – since the total filling volume is known, one can look up the densities of the gas and liquid phases at that temperature, and then use the following formula to determine the volume occupied by each phase. That is, (gas phase density × gas phase volume) + (liquid phase density × [total volume – gas phase volume]) = total fill volume. Once the volume is determined, the liquid level can be calculated based on the size of the tank. Follow-up question: If this approach to calculation is correct, it means that when the tank is fed or discharged, as long as the temperature remains constant, there is only a change in the liquid level inside the tank, with no change in pressure. Is that really the case?