SharePoint 2013 Calculating Scores

I am working in SharePoint 2013. I am trying evaluate the work of my staff and will be using a list to track this and want to assign a grade to the results. If the person gets a 4 out of 5 I want them to receive 80%.

The way I'm doing this is with a list that contains 5 questions with a drop down menu and the response are Yes, No, N/A. There is a second 5 columns that converts the drop down list selection to a 1, 0, or "Empty Record". Yes=1, No=0, N/A=Empty. This is an how I'm converting the responses to numbers: =IF([Question1]="Yes",3,IF([Question1]="N/A"," ",0))

So a response of (Yes, Yes, No, Yes, Yes) should convert to (1, 1, 0, 1, 1) which should = 4 out of 5. (80%)

The problem is how to calculate the grade so that if the an empty record doesn't factor into the calculation.

So a response of (Yes, Yes, No, Yes, N/A) should convert to (1, 1, 0, 1) which should = 3 out of 4. (75%)

I've got the conversion of the responses to numbers down pat, just can't get the calculation of the grade to work.