jet boats tend to put out a crappy wake. Try going at different speeds and watch the wake to figure out when it's clean. Once you know what speed makes the best wake, you can drop the rope out the back of the boat. When the handle reaches the clean part of the wake, that's the length to try. Hopefully it'll be in the 50' - 65' range. You don't want to be too close to the back of a jet boat cuz the jet makes a very turbulent wake.
Hope this helps!