weighted scoring formula to exclude not applicables

Hi All,

I am stuck big time with this. The attached shows a report to collect data for an evaluation. A score of 1 is entered against each category depending on the outcome of the call. The part where I am stuck is if a category is N/A I want the total percentage score to not take this into consideration. As an example. All the categories could score excellent with the exception of telephony skills which is N/A. In this case, the score is 96% but I want the calculation to only include those categories where the whole section is not N/A.

Really hoping that someone can assist with this. I know there are enough bright sparks out there to dig deep and help me through this!!!
Thanks in advance guys