A possible solution: Adding scores to yes/no and multiple choice questions would be a nice start.
I believe it wouldn't be too difficult to do: Yes/No/Not answered question types could be paired with numeric values: yes = x ( where x can be an arbitrary number chosen by the user ), No = 0, Not answered removes the question from the percentage calculation but behaves like No for total score. The same, but with a wider range of values, could be done for multiple choice questions. Other question types ( Long text, Short text... ) could be left as they are, with no numeric value.