Rest the end of a straight 8' - 2" X 4"
beam on the door sill and
bring it to a level position.
Measure from the face of the house out at even step intervals (refer to
total projection table) and measure the drop to the ground or finish grade.
Remember,
you should be more interested in where the bottom step will be, rather than just under the
doorsill.
When measuring for width, be sure choose a
width that is wide enough. Most three foot doorways can easily take a
width of 5 or 6 feet. Remember, railings come in from sides about 4
inches. With shutters or side lights steps can be 7 feet wide, or wider.