Sorry for responding now Hip - 3, then divide by 6 and add 1" seam allowance E g hip 40 - 3" = knee circumference ie 37 37 ÷ 6 = 6.2 6.2 + 1" seam allowance to join both sides= 7.2
It is dependent on the fit you want to have around the knee and also do you can walk freely. Hip circumference - 3" will give you the perfect knee circumference.