Nail Polish method to measure Nail Growth

How do we design a method to calculate nail growth using Nail Polish mark method? In this picture one photo is having full nail covered with nail polish, whereas in other one the movement of nail polish has taken place indicating nail growth. Can we calculate the growth using ruler? What can be other ways to measure nail growth?

Questions emerging from the discussion:

  1. Why is the nail polish not even on tip in second photo?
  2. How do we measure the nail growth using nail polish?
  3. If we do the same with 3 year old, 16 yr old and 60 yrs old what will be the difference in nail growth?

The photo is taken by Cubist Khushi (20 yrs old), the photo is taken at the interval of 3 days as claimed by Khushi. We can see this nail growth taking place in 3 days time.