A method for generating a trimming setpoint for an ophthalmic lens for it to be fitted in a surround of a half-rim spectacle frame, includes the steps of: a) acquiring the planar coordinates of a plurality of first points characterizing the shape of a first longitudinal profile of the surround; b) determining the spatial coordinates of second points of a second longitudinal profile, obtained from a projection of the first longitudinal profile onto a predetermined reference surface, c) calculating the perimeter length of the second longitudinal profile, d) determining the spatial coordinates of third points of a third longitudinal profile, obtained from a projection of the first or of the second longitudinal profile on a curved surface deduced from a shape parameter of the ophthalmic lens, e) correcting the spatial coordinates of the third points, so as to equalize the perimeter length of the third longitudinal profile with the perimeter length of the second longitudinal profile, f) deducing the machining setpoint from the corrected spatial coordinates.