There is another consideration in line weight. Braking. You might get away with 150 lb lines as far breaking strength but you might want to go up in line strenght to take advantage of the braking effect of the larger diameter line. This is a major characteristic. If you are flying long lines with large diameter you will be slowing your kite down and that can good in heavy winds. Another solution is to usw extra large leaders 20 feet with large diameter.
It also works on the other end. In light winds use the thinest lines possible.
Take note of the belly in your line as it goes out to the kite. That belly is line braking. Too much is very bad as it will act dead zone or backlash on your inputs.
If the line is .06 inch diameter versus .03 in. diameter and the length is 1200 inches or 100 feet. There is in effect a 36 in sq extra brake on the kite. That is substantial.
Math is (.06-.03)*1200=36
YOur referring to the drag the line has which causes a "braking" effect. First off your math is wrong because your dealing with a rod shape made up of multiple rod shapes with an uneven surface, but we won't get into the math. Not only do you have to know the right circumference but the surface deviation and a bunch of other factors ,like depth of the cup inbetween strands, coating method (strands coated before braiding will have more drag then coating after braiding) and surface area presented to the wind to get a drag coefficient. Too much of a headache for what we do so don't bother with it.
There are better ways to add drag and slow your kite than line thickness. Breaking strength is way more important than drag of the lines.