I was wrong before, the goal he was after was in fact to change the target AFR value. I do not yet understand what would be the difference of a short spread VS a wide spread if the midpoint is the same number.
It seems that changing values to shift mid point will change the sensors target AFR. With the modified midpoint value of .47 the target AFR is 15.1, remember the lower the midpoint value the leaner it will read.
Have a look at this chart
http://cherrypicker.tripod.com/id12.html
Using that chart, if we take the stock mid point of 0.4885v the target AFR is reading is 14.8.
You can see the richest value possible to be read by the narrow band is only
14.4 AFR, much to lean to tune WOT.