I do use an exact length, I fill the reel right up, if you take the starter cover off and flip it upside down, turn the starter reek clockwise until it starts getting stiff, use your third hand to feed the rope in from the outside of the starter into the center of the reel and tie it off, then use the spring tension to pull in as much rope as possible.
Putting a long rope into the starter will reduce wear on it as it is not hitting the end of the reel with a decent thud every time the rope is pulled.