If you have 20 single answer questions, you can use
Qualtrics Scoring and assign 5 pts to every answer option.
If you have a mix of questions, you can pipe the answers to all the questions into a script (web service or JavaScript) that would determine which questions were answered and calculate the percentage.
Another approach would be to have 20 branches in your survey flow at the end of the survey to check if each question was answered. Each branch would update an embedded variable using a Qualtrics math operation.