We have found that having a threshold mark is just not effective. Students will do the quiz over and over and over to get the score without paying attention to the video or making good notes. 

Make homework be about producing good, quality written work, following the guidelines in the student checklist which can be downloaded from the useful resources page. It almost doesn't matter what score the student gets, if they produce good homework, then the results will follow.

This takes the pressure off the students for whom maths creates anxiety, and means that as long as they complete the checklist, their homework is done and done well.