I ran this test several years ago on my scope and tried to explain it in a post but Bryan Litz video makes it much easier to understand. I used a long tape measure as well.
https://www.youtube.com/watch?v=12Wf0Cuwwi8&feature=youtu.be
https://www.youtube.com/watch?v=12Wf0Cuwwi8&feature=youtu.be