Kazan said..
So what's a good length overall for say 15 to 20knots? Is there a calculator out there that would tell you what would be optimal line lengths given wind speeds, and other kite factors (size, shape, rider's weight, etc)? Might have to get the maths myself and make one!
Every kite is different. If the kite co. has done its R&D as it should then it will release the kite with the appropriate length lines for that particular kite & size of their own design.
Adding extensions to a well balanced standard issue length will increase the bottom end, make it fly slower, etc etc.
But dont be fooled into thinking that just because your choice of kite comes with 20M lines that it will make it the equivalent of your mates kite that comes with 23M lines.
Extensions should be just what they say they are - extra length ( to any standard length) to make your kite work a little better when the day isnt providing quite enough power.