The reference angle is always with respect to the x-axis, so you wouldn't subtract it from 270 if it's in the 3rd quandrant. It should be the difference between 180 and 249.3. In other words, the reference angle is the angle between 0 and the angle for quandrants 1 and 4, and between 180 and the angle for quandrants 2 and 3. See a pretty picture for details.