Exterior Window Trim and Rough-In Sizing
Pick out your trim for the window and for the frieze.  Decide on head flashing. Decide on the jamb reveal.  Make your best guess for the window position. Then frame the rough opening 3" extra wide and 3" extra tall. Then fill the RO down to size by screwing (not nailing) 1x stock to the jacks, header, and sill.  Center your sheathing over the RO so that a single 4x8 sheet covers it entirely. Nail it to the 2x framing members, but not to the 1x.