Card PD - Pupillary Distance

CardPD is a patent pending system that allows the user to get accurately the own PD measurement, wherever he/she is, not requiring the use of a ruler or additional objects on the person's face, requiring only an Android device and a card of any type.
This method is based on solid mathematical rules taken from the projective geometry and computes the PD accurately.
In opposition to the other methods, CardPD does not use the camera and no clicks on images are necessary.
INSTRUCTIONS:
FIRST STEP - Calibration
Choose a rectangular rigid card such as a visiting card or a credit card, and place it on top of the screen and line it up between the two cyan vertical lines.
Move the right vertical line with the Left and Right buttons so that the card fits inside the two lines.
HINT: Use your credit card in a position as illustrated in the App.
SECOND STEP - Measurement1.
Sit comfortably in front of the device, hold the card with your hands and close your left eye.
2.
With your open eye, fit the card between the two cyan lines on the screen moving the card forwards and backwards.
3.
Stop moving, when the card is perfectly fitted.
From now on, keep the card still.
4.
Now, open your right eye and close the left.
5.
Your PD is given by the intersection of the left edge of the card with the numbered line in the picture.
HINT 1: You can invert the process by starting it with your right eye closed.
To do so, click the FLIP button.
HINT 2: Check all the process by clicking the HELP button for a more visual tutorial.