A method for determining a fully extended position of a screening body (14) of a screening device (12) for a roof window. The screening device (12) comprises a control unit comprising a data storage device, an electric motor (18) comprising a tachometer (181), a roller tube (15), a screening body (14) and a first and a second spring element (164, 174). The method comprises the steps of driving the screening body (14) from a fully retracted position to a fully extended position, in which the spring elements (164, 174) are tensioned to a first tension level, T1, stopping the electric motor (18) at a point at which the spring elements (164, 174) are tensioned to a second tension level, T2, above the first tension level, T1, measuring the number of revolutions, Rd, of the roller tube (15) necessary to drive the screening body (14) to the said position at which the motor (18) is stopped, storing the measured number of revolutions, Rd, in the data storage device, measuring the number of revolutions, Rb, of the roller tube, that a release of a tension corresponding to the difference, ΔT, between the first tension level, T1, and the second tension level, T2, will cause the roller tube (15) to move back towards the fully retracted position, storing the number of revolutions, Rb, in the data storage device, and calculating and storing in the data storage device a value R=Rd−Rb.