Inherit scale has to be set so that when I scale individual faces of parent mesh, the dupli objects are kept the same size as scaled faces of the parent mesh.
In these screenshots, the parent mesh object has 3 differently scaled faces (orange) and the child object blue.
Inherit scale is 0.25 and it's too small, blue duplicates are much smaller than parent mesh object faces.
Right side: Inherit scale is 0.5 and it gives the desired result, blue dupli objects are exactly the same sizes as parent mesh object faces:
Given any shape, how do I calculate the correct inherit scale value to make the dupli objects the same size as the parent object's faces?