what? its the rope and handle length. doesn't matter where its attached dont get confused its the rope and handle length usually 70 ft rope + 5 ft handle, the 70ft rope usually has some 5ft take offs if you want to go shorter and get across the wakes easier(narrower)
with a 75 foot rope attached to the tower and the tower being roughly 6 feet above where it would attach to the back of the boat you are going to lose only about a half a foot in your actual length from the boat.
just use the old a^2+b^2=c^2
except in this case since you are given the hypotnuse (your rope) which is c^2 and the height from the back of your boat up to the tower which is b^2 you are going to use c^2-b^2=a^2 where a^2 is your distance from back of the boat
although this varies because your tower tow point is not directly above the boat