its actually not "degrees" but inches. it goes like this 15 Inch curves have a diameter of 30 inches, 18"R = 36" , 22R"=44" and so on so forth. degrees is usually how far along the curve goes. before it straightens out again. I would not reccomend 15 inche radius because almost nothing will run on it or look good. His track plan seems to be using 22 on the outer and 18 on the inner curves.