We now find the center of the left flap (see below) by situating a ruler diagonally and making a short mark near the middle. The ruler is aligned with the outermost crease at one end (upper left, in the image) and at the opposite corner of the flap.

Note: Remember that the enclosure has been flipped and that the outside is facing up. The instructions assume it has been flipped only horizontally, as indicated by the white arrow. This means it is horizontally reversed, that the flap we are calling the "left" one is on the right. We could have flipped the enclosure vertically too, to maintain left-right orientation, but that would potentially be even more confusing.