ASTM E2112-01* and AAMA 2400-02* recommend the use of a minimum 9" wide flashing membrane beyond the rough opening. Grace recommends a 12" wide flashing membrane for proper waterproofing of the window sill.
Sill Flashing: Rough Opening width + (2 x flashing width)
Jamb Flashing: Rough Opening height + (2 x flashing width) - 1 inch
Head Flashing: Rough Opening width + (2 x flashing width) + 2 inches