A method of determining a first relative rotation of coded data on a print medium to be printed on by a mobile device that has a printer, a first sensor and processing means. The print medium has a substrate defining first and second opposite faces, the first face bearing coded data. The method involves capturing a first image of the coded data when the print medium is at a first position using the first sensor, determining a first relative rotation of the coded data with respect to the print medium from a known physical orientation of the print medium and the first image, using the processing means, capturing a second image of the coded data when the print medium is at a second position using the first sensor, determining a second relative rotation of the coded data with respect to the print medium from a known physical orientation of the print medium and the second image, using the processing means and calculating a third rotation from the first and second rotations using the processing means. The third rotation is a more precise measure of the first relative rotation than the first relative rotation or the second relative rotation.