The invention relates to a method for determining a level of a material interface in a tank, by means of a radar level gauge system comprising a transceiver; a probe for guiding a transmitted electromagnetic signal towards the material interface. The probe comprises a first plurality of reference impedance transitions located above the interface at known physical distances from a reference position, and a second plurality of reference impedance transitions located below the interface at known physical distances from the reference position. The method comprising determining electrical distances to the first and second plurality of reference impedance transitions based on a signal reflected by the reference impedance transition, determining, a first and a second approximation function relating the first and second sets of electrical distance values to the physical distances; and determining the level of the material interface based on the first approximation function and the second approximation function.