Abstract: A whipstock assembly for milling an aperture in a casing of an oil, gas or water well, the whipstock assembly having a whipstock with a deflection face (6) for deflecting a milling head of a milling device to cut through the casing. The whipstock has a plateau portion (7) raised above the plane of the deflection face to support the milling head as it engages the casing. The plateau can induce the milling device to pivot around one of the milling heads during some of the milling operations, which optimizes the stress applied to the heads of the milling device.