Checkbox conditions

I will try to clarify,

I have scoring associated with question, and at the end of the form all questions with be averaged.. I have 6 responses numbered 0-5 ( with response 0, no debt.). I then have to take the value (count selected) and subtract it from 5.( the scoring was derived by another team) This is what I have:

The problem is I can't figure out where the to add the validation condition to my original calculation

Note: I assigned the value of 5 to 'no debt' so that when ran through the cond(count-selected) it would return 0. However, without the validation condition, users can still select '0' ie. no debt, and the other options .

Thanks for the advice/suggestions