I found a solution (albeit a somewhat clunky one).
1) Create a slide that displays results (not a Results Slide).
2) I display the value for NumberCorrect. Duplicate the slide (we'll call this Results1 and the duplicate Results2).
3) Set up a trigger so that variable PercentCorrect is set to the value of NumberCorrect when the Results1 slide starts. Don't display PercentCorrect yet.
4) Make a button called CALCULATE SCORE. Create a trigger that divides PercentCorrect by total points possible.
5) Make a trigger that moves to Results2 when user clicks CALCULATE SCORE.
6) Create a trigger that multiplies PercentCorrect by 100 when the timeline starts on this slide.
7) Now you can display the PercentCorrect variable along with the NumberCorrect. Since you duplicated the page it will appear instantaneous (as long as there aren't any transitions).
