Average your quiz scores.
Quiz averages are points earned over points available, which handles a missing or partial quiz correctly. The more useful output is forward-looking: multiplying your target by the final points available and subtracting what you already hold gives the points still needed, and expressing that as a percentage of the remaining quizzes tells you whether the target is reachable at all.
Current average
Current average = points earned / (quizzes taken x points per quiz) x 100
Required future average
Needed average = (target x final points available / 100 - points earned) / (remaining quizzes x points per quiz) x 100
Because with few quizzes each carries large weight. A 5 out of 10 among seven quizzes costs over seven percentage points of the average; among twenty it costs about two and a half.
This assumes a common total. With mixed totals, sum the earned and available points directly rather than averaging percentages.