Refund policy for a risk-free purchase of a short online course [closed]

I am building a website for celling and teaching online courses of my own making. Consider a course that consists of 50 videos for a total of seven hours. I would like to offer a refund policy that will alleviate the potential student's doubts regarding the possibility of a poor quality course. On the other hand, I would like to protect myself from people learning the whole course and requesting a full refund (though I wonder how many people would go for this if they really got value from the course).

I am thinking of offering a refund policy, whereby one can only obtain a refund within one week of the purchase and only if they have watched through less than 70% of the videos. Would such a refund policy make for a risk-free purchase?