going back to the days of typewriters, business letters written on 8.5" x 11" letterhead start approximately 1.5" below the letterhead. left margin = 1", right 1.25". however, many companies have their own letter style, often because of the letter head design.
europeans use a slightly longer and narrower paper [a4, 210mm x 297mm]. their margins tend to be wider [more white space]. [sig][/sig]