Quadrangle rule is valid when above the transition level, which kind of depends on what FL you plan to fly and the QNH. In some scenarios if the QNH is right or wrong whichever way you look at it the transition altitude or the quadrant level you need to fly is not available for your heading and the FL that you plan to fly, in which case you need to pick the next FL avaible for that heading, which in turn gives you the Transition altitude.