read this thread and then read the links that dave williams posted about being "that guy". that will give you a decent foundation.
rope lengths depend on a number of things, speed youre riding, wake size, and preference. i ride at about 60 feet at 20.5. alot of riders ride much further out at higher speeds. if you wanna ride out as far as possible on the wake you have then
1) bring your boat up to speed
2) then let the rope out to the point just before where the wake starts to wash out(curls over)
however i dont recommend beginners riding at 80 feet and 24-25 mph when they are learning. make it a little easier on yourselves and ride slower and shorter. the falls will hurt less and it will take less effort to clear the wake.