Garment neck circumference
  
Measure the length from the neck line in the front and back part of the garment pattern. Measure at the front from the center front point to the neck-shoulder point( do not measure from the button extension line). At the back measure from the center back point to the neck-shoulder point.
Fill in the whole length of the neck line, so multiplicate the total measured value by 2.
You can measure this in Patternmaker with the Set/Measure Distance command in the Point menu(if available in your version). 
Or you could print the pages of the neckline and measure them manually with a measuretape.