The Air Flow Sensor and the engine RPM (which together would be the load)
are the 2 main imputs to the ECU.

While driving in open loop if a person watches the 02 sensor output they
would be able to see how well their AFM is adjusted. As Darrell pointed out
the spring in the AFM change tension with age. I drove open loop in my van
the other day and the 02 sensor was reading .8 or more. I adjusted the AFM
spring tension and got the open loop 02 down to .7.  I was trying to solve a
warm idle surge problem.

It would seem quite important to check your open loop 02 if a person was
going to drive in open loop for any significant amount of time. Too lean
could cause engine damage. Too rich may cause poor milage, plug fouling,
ruin your cat. converter.....

I'm not an expert, just my 2 cents worth,

Mark Ingalls