1. Roll up your pants and place the foot on the A4 sheet. Make sure the foot and the A4 paper are touching the wall edge to maintain alignment.
2. The area around the A4 paper should be dark (eg. a dark coloured floor)
3. Hold your phone in landscape mode and ensure the top half of your foot is clearly in focus.
4. Ensure the mobile is below waist level for the image to be processed correctly

Check if the A4 sheet, your foot and the wall edge are aligned.

Any misalignment will affect the accuracy of the measurement.