Are the inside returns drywall?
Usually you would measure jamb to jamb if you are setting inside the drywall and the replacement frame is deeper so that happens or measure stud to stud and cut back the drywall to accommodate if the siding isn't the smaller measurement.
Usually you would measure jamb to jamb if you are setting inside the drywall and the replacement frame is deeper so that happens or measure stud to stud and cut back the drywall to accommodate if the siding isn't the smaller measurement.