What length lines?
You need to decide on a line length. Rule 1 permits any line length, but there will obviously be a range of lengths within which your model will perform best. The longer the lines that your model will safely fly on, the greater will be the time difference between your fast and slow runs and therefore the higher will be your final flight score. The shorter the lines, and the easier it is to make a successful arrested landing!
All lengths are measured from the front face of the handle, to the center of the model. So the actual line length will always be less than 60 feet. Don't forget to subtract the length of your loadouts and any extension forward of the handle face. The only limit is the flight circle itself which has a fixed radius of 60 feet. Read the rules to find out full details. Don't obsess about this if you are starting out, but if you fly in the nationals they may be checked.