youre missing the point. everyones ideal rope length is different. you may like 80', another guy might like 60'. doesn't matter unless there is a law in your area about the max length of rope you can have.
I teach wakeboarding lessons and I pretty much start kids at about 55' and adults at 60 to 65'. But thats all depending on the spped at witch you are going and how clean the wake is at the length. If the wake is cleaner 5' back then legthen your rope.
like j-one said, different for every boat, speed, skill, etc. beginners, just find where the wake is the best. mine is 48' at 14 mph, i think. thats what we have the most fun at, anyway, hope that helped!
