How to add the values assigned in the multiple choice widget?

I have enabled the "Add Calculations Values" option to my Multiple Choice Widget. I would like to know how to calculate the assigned values. Each item has an assigned value that particular box should equal to 15. As soon as it equals 15 the user should no longer be able to select another checkbox even if they haven't reached the maximum amount of checkboxes allowed.

Thank you

You need to us a calculation widget to calculate the values: https://www.jotform.com/help/259-How-to-Perform-Form-Calculation-Using-a-Widget 

And you can disable a field based on the total calculation: https://www.jotform.com/help/371-How-to-Enable-Disable-Fields-Using-Conditional-Logic 

Let us know if you need more help.