During the first several months of the project I made ink footprints that I measured to see if my arch was improving. I used the measurements to calculate something called the Plantar Arch Index.
The plantar arch index establishes a relationship between the central and posterior regions of the footprint, and it is calculated as follows:
A line is drawn tangent to the medial forefoot edge and at heel region. The mean point of this line is calculated. From this mean point, a perpendicular line is drawn crossing the footprint.The same procedure is repeated at the heel tangency. Measurements are obtained of the width of the central region of the foot (A) and of the heel region (B) in millimeters. The plantar arch index is obtained by dividing the A value by B (PAI = A/B).
Basically, this index is comparing the width of the heel to the width of the middle of the foot in standing. A lower index value means a higher arch.