In the method file, define your peaks to identify. Check the one you want to see in the calibration table (check the calibration column)
In the calibration part, check the External calibration. Then, you will see your peak used for the calibration.
Search in the screen, you have a level number, choose 1.
In the calibration table, you will have then only 1 level to enter.
Please read the user documentation (or on-line help) if it is not clear, everything is explained in it with screen shots.
hopefully it helps !