The most accurate way to measure is to measure from the center of the box-spring, the width and then the length. Because the corners are rounded on the box spring, that is why you measure closer to the center. As for the drop, take the tape, (careful not to push too hard on the box-spring), then measure to where you would like the bed skirt to sit, breaking on the floor or be flush with the floor.
This article also appeared in the Toile Newsletter, Fall 2011