Trial and error. Once you get it right - save it as a template.

Fold a sheet of paper the way you will fold your printed letters. Insert it
in the envelope, from which you have removed any plastic window insert. Draw
a pencil line around the outline of the window. Remove the sheet of paper
and use a ruler to measure the position of the window on the sheet. Then use
paragraph formatting or a frame or text box to position the delivery address
there.

Then, as JoAnn says, save the resulting document as a template. For more on
letterhead templates in general, see
http://sbarnhill.mvps.org/WordFAQs/Letterhead.htm