Z-depth on already distorted mc

I’ve got an isometric view of a flat surface. On this surface there are various characters running around and I obviously need to control the depths of each object.

My flat surface needs to be used in two views: top-bottom + isometric.

The original mc is generated in the flat top-bottom mode. For the isometric view, that same mc gets distorted: this way the whole XY coordinates system gets distorted too.

Have a look at this (updated).

So i’d need to create this extra coordinate (is it the classic Z-depth?), and manage to gather information about it…

So hard to explain…

Does anyone know what the formula would be to calculate the depths of each character (red lines)?