A List Score field calculates the percentage of either the "yes" or "no" that is selected from the total number of options within a singular Yes/No List field or all of the Yes/No List fields contained within the form. 

Name your field. The Limit to list field drop down will allow you to select a specific Yes/No list field or if you leave it as n/a it will count all of the selected values from all Yes/No list fields. Count options with value will determine which value (Yes or No) to count.