Setback from residential dwelling unit. Except as governed by a greater setback required herein, all billboards must be set back a minimum of 150 feet from the boundary of the R1, R2, R3 and R4 Zoning Districts and 500 feet from the boundary of any property on which a residential dwelling unit is situated. Moreover, the applicant must establish that the billboard will be screened such that the face of the billboard will not be visible from an existing residential dwelling unit. With respect to such setback as applied from any existing residential dwelling unit, the 150-foot setback may be reduced if the owner of such dwelling unit agrees in writing to the placement of such sign as memorialized by an easement filed with the Chester County Recorder of Deeds.