It is just an exercise, no one is being forced to do this. However, you need to start somewhere and 15 ft would be my answer as well.

I was given several lines for X-Mas including 2 for my 8wt. The floating peachy 444 was trimmed several ft for nymphing. Now, I'm going to cut off 9 ft from the 444 SL QD 425 gr line. It is rude to return gifts but not to modify them. Hitting the Salmon River for the first time on Saturday.