All measurements, tests, and analyses of the characteristics of waters and wastes to which reference is made in §§ 50.21 and 50.22, shall be determined in accordance with "Standard Methods for the Examination of Water and Sewage," and shall be determined at the control manhole, or on suitable samples taken at the control manhole. In the event that no special manhole has been required, the control manhole shall be considered to be the nearest downstream manhole in the public sewer to the point at which the building sewer is connected.