Calculating NPS (or tabulating scores in a form) for a survey

If you are using checkboxes for your survey, you can set values for the individual items in a checkbox

Adding a hidden field as a total and using Javascript to keep the hidden field up to date will allow you to sum totals of a form's quiz

If you're using checkboxes:

If you're using radio buttons

Consider posting the hidden field to a custom field on the contact