Hardness SHORE D 40

The measuring of the shore index works as follows: The Shore-​measuring an instrument is nothing but a needle, that comes out of the Shore-​Meter. The needle goes into the material and shows you the hardness of the plastics, but it says nothing about the flex. The flex is measured by the elastic module.

As thicker the material is made, as less is the flex. This means:
Thicker material — less flex — same shore hardness — same flex module.
Result: You only can determine the flex by trying the boots with
different plastics. No other way is possible.