If you have commanded an AFR of 12.5:1 across all engine speeds (RPM Points) at WOT and you perform a test run on the dyno or track and see that your resulting AFR is 13:1 your MAF curve must be adjust to correct for this.
In this case an AFR of 13:1 is approximately 4% leaner than commanded. To correct for this you would highlight the cells corresponding to airflow and add 4% to the values in those cells and retest?
The question is?
How did you know it is 4% not 5 or 6%?????