Abstract: The invention relates to a deviation calculation rule of the circular type for the monitoring of a two-dimensional deviated well path. The rule consists of a support plate (10) on which is marked a circular dial with two semi-circular scales (G1, G2) graduated in degrees for locating the inclination angle at the entry point and the exit point of a section of path, at least two rotary disks (12, 121) each marked with a plurality of concentric circles (C1 through Cn; C1 through C′n that each correspond to an inclination gradient for a specific drilled length (Gbu), at least one bi-sectoral disk (14) with, on its edges, scales (E1 through E4) used to locate the various Gbu circles on the disk being used (12), a first sliding system (16h) allowing for a direct reading of the horizontal deflections and a second sliding system (16v) allowing for a direct reading of the vertical depth deviations.