help finding percentage of several fields

I have 3 columns of check boxes. All 3 columns total up the amount of checked boxes selected at the bottom into 3 separate fields #yes #no and #N/A. For the sake of this question lets say they total to 11,2,0. Now i need to get a percentage based on the yes column (85%) in another field.

Im having a hard time figuring out how to calculate this.

thanks