Method for determining a position of an optical lens member placed on a lens blocking ring (22). A reference system, blocking ring data, and optical lens member surface data are provided. Position parameters are provided defining a position of a reference point of the surface of the lens member with respect to the main plane of the reference system and an orientation, about the main axis of said placed surface at said reference point. The position of the placed surface is determined according to the position parameters. During a repositioning step (S6) the placed surface is virtually translated and rotated. During an altitude determination step (S7) a difference in position between the blocking ring and the placed surface is determined. The repositioning and altitude determining steps (S6, S7) are repeated so as to minimize the difference in position between the blocking ring and the placed surface.