SPIDER experiment includes an RF inductively coupled plasma source working at 0.3 Pa of gas pressure (H/D) where plasma is generated and heated by eight RF drivers, fed by four RF circuits. A single RF circuit is composed of two drivers connected in series, attached to a capacitive matching network, fed by 200 kW 1 MHz RF oscillator through a coaxial transmission line. The knowledge of driver impedance in different experimental conditions is a valuable window for understanding the characteristics of generated plasma. Direct measurement of driver impedance in SPIDER is not possible, it has been estimated via the development of a suitable electrical model of the RF circuit using as input, measurements at the oscillator's output. This paper reports on the progress in the modeling of the SPIDER RF circuit and discusses a procedure to estimate the driver impedance exploiting also the recently available experimental measurements from the dual directional coupler. The impedance obtained with this approach, derived from measurements obtained in a recent SPIDER campaign, is reported for various operating conditions, such as RF power, gas pressure, plasma grid current, cesium injection rate, type of gas (hydrogen and deuterium), etc.