To get holes for overhead fixtures and wall outlets right, measure from the point where the side edge of the panel will be to the near and far sides of the electrical box. Then do the same again, measuring from where one end of the panel will rest. Transfer the marks to the drywall sheet, then pencil in the outline of the box. You can then drill out the corners and cut between the holes with a keyhole saw.
KAY HOLMES STAFFORD