PC version here
To ensure the accuracy of the measurement, we strongly recommend that you calibrate it before you use it.
Take a standard size credit card for comparison, select the option "Standard Credit Card" to show the ruler adjuter, enlarge or reduce the scale until you are sure that the scale of the ruler is the most accurat. Remember to save the setting, so you can use the ruler directly next time. You can use anything for comparison as long as you know its size.