Rope length depends on the wake your boat makes.
I have a Tige Switch V and at 20mph my son uses a 55’ rope. (He is learning how to jump w2w so he likes the shorter rope)
This puts him about 8’ behind the center point of the wake.
When I ski (tandem ski) at 26mph I am just behind the center point (where the two wakes start) so need to use a 65’ to 75’ rope.
You will notice that the wake will change in both size and length as the speed of the boat changes.
I have read that you should bring your boat up to speed, and let out you rope until it hits the spot you want to wakeboard at. There is no magic #.
Hope this didn’t make to many more question for you