A data processing unit configured to hardware calibrate a gaze tracking device with respect to a display unit, characterized in that the data processing unit comprises: a data output interface configured to produce control data (G-Ctrl) for presenting at least one graphical guide object (G1, G2, G3) on an active area of the display unit, the at least one graphical guide object (G1, G2, G3) designating at least one distinctive feature (121, 122; 123) of the gaze tracking device reflecting how the gaze tracking device may be positioned (x1, x2) on a first side of a frame of the display unit, and a user input interface configured to receive at least one user input movement command and a user input confirmation command, and the data processing unit is configured to: produce the control data (G-Ctrl) such that at least one of the at least one graphical guide object (G1, G2, G3) moves on the active area in a first direction parallel to the first side of the frame in response to the at least one user input movement command, assign a position value (Px) designating an actual position for the gaze tracking device on the first side of the frame based on a current position of the at least one graphical guide object (G1, G2, G3) on the active area, calculate an offset value (X-OFF) based on the assigned position value (Px) and a known measure of the active area, the offset value representing a distance in the first direction between a well-defined point of the gaze tracking device and a well-defined point of the first side, determine a width measure and a height measure reflecting a physical extension of the active area; and store, in response to a user input confirmation command, the offset value (X-OFF), the width measure and the height measure for use when determining a user's gaze point on the active area. a data output interface configured to produce control data (G-Ctrl) for presenting at least one graphical guide object (G1, G2, G3) on an active area of the display unit, the at least one graphical guide object (G1, G2, G3) designating at least one distinctive feature (121, 122; 123) of the gaze tracking device reflecting how the gaze tracking device may be positioned (x1, x2) on a first side of a frame of the display unit, and a user input interface configured to receive at least one user input movement command and a user input confirmation command, and produce the control data (G-Ctrl) such that at least one of the at least one graphical guide object (G1, G2, G3) moves on the active area in a first direction parallel to the first side of the frame in response to the at least one user input movement command, assign a position value (Px) designating an actual position for the gaze tracking device on the first side of the frame based on a current position of the at least one graphical guide object (G1, G2, G3) on the active area, calculate an offset value (X-OFF) based on the assigned position value (Px) and a known measure of the active area, the offset value representing a distance in the first direction between a well-defined point of the gaze tracking device and a well-defined point of the first side, determine a width measure and a height measure reflecting a physical extension of the active area; and store, in response to a user input confirmation command, the offset value (X-OFF), the width measure and the height measure for use when determining a user's gaze point on the active area.