Looking at the diagrams on this site it appears the motion detection with the doorbell mounted at 4’ is consistent vertically from the doorbell out to the detction limit, in other words, 4’ all the way out is the detection limit. The lower detection limit is then angled downward as you go further away. It is basically a triangle, flat at the top, angle down according to your installation pics. If installed where there are steps, the wedge kit will change the uper limit so that the entire detection triangle is now angled down. Instaling different wedges changes this angle.
With all that, what is the angle of the lower limit of detection from the doorbell. Knowing this one could use simple geometry to calculate the correct wedge rather than trial and error.