Measuring verts is a tricky topic. There are so many methods.

Measuring the height your head travels should be close to your real vert, which in this case he is 5'9 and the rim is 10 ft and let's say he got top of head around 2 inches above rim. That would be a vert of 53, so 51 sounds right.

But for me my head is 3.5 inches away from touching my ceiling 27 inches away equalling to 23.5 but according to flight time it's 17 inches.

Also using vertec does not really seem accurate as you measure flat footed but when you jump you toe off, so maybe starting reach should be measured on tip toes.