The variable for the percentage that the user got is cpInfoPercentage. I agree, it should also have Quiz in it, but hasn't. That has confused a lot of users up till now.
Download my descriptive list with all system variables, link is in this post:
http://blog.lilybiri.com/system-variables-in-captivate-6
You can show a message (text container) based on the value of cpQuizInfoPassFail.
Here is another post that could be useful:
http://blog.lilybiri.com/intermediate-score-slides
Be sure I don't earn a dime with that blog, only started it to avoid always having to explain the same things in the forums. 
Lilybiri