You are not quite seeing the picture. The illustration you present is a graph of mixture. The point of the graph is to show the area that must be found through fuel control.
What you aren't realizing is that when you are "IN CONTROL" you are in a state of manipulation where the mixture is held to that ideal area. "In CONTROL" means that the same mixture is had at 20% as at 80%. The difference is the remaining ability to correct.
All of this control is on top of the mechanical/hydraulic system that really does all the work. The difference between 20% and 80% is in the state of the mechanical/hydraulic decision. Whatever that decision the electronics tunes out the right mixture as long as the base mixture is within the correction window.
The reason I suggest 35% say, is that all mixtures done closed loop and all instantaneous mixture changes are done with the mechanical/hydraulic decision. I prefer slightly rich in these situations. Remember that once warmed the closed loop system keeps the mixture dance at exactly the same point in any steady state (the rich lean correction takes around a half second at best so steady state is anything happening slower that that).
Bosch Master, ASE Master, L1
33 years MB technician