How to calculate a percentage with ambiguous numbers?

Evaluation forms for a class I teach ask the student to circle a number between 1 and 5, but there is also a N/A column.

I want to know what percentage of my students enjoyed the class. If they only circle numbers, the calculation is easy. But if someone circles a NA, how can I calculate that? It’s not a zero. Thanks.

EDIT: Imagine there are 20 opportunities for someone to circle a 5. If 5 students circle all 5s and 5 students circle all 4s, then the average evaluation is 90% (assuming a 5 is 100 and a 4 is 80). But what number value is a NA?