Custom message based on average result

I want to tweak the plugin slightly for the results display. I want to modify the php such that it shows a different specific message if the average score is between 1 to 2, 2 to 3, 3 to 4, and 4 to 5. So for example, instead of showing “Average score: 4.23 out of 5” I would like it to say “You scored an A”, and if the average rating is 2.98, I would like it to say “You got a C, sorry!” and so on. Where in the php should I modify the code to achieve this? It appears like this might be achieved from expand_ratings_template() but I was wondering if there was a hook I could use instead.