I was going to say 75 skill and 25 luck... Especially if you are just on the edge, like anywhere in the 70's. If you score 99, well that is probably 95% skill... Because that is amazing!
I've gone into all of my exams just hoping I'll pass and knowing it could go either way. I never consistently score 90+ on my MCQ's when I review. The exam I felt the absolute worst about was my best score. Go figure! Good guesses?