Method for assembling a mould having a casting cavity for casting a semi-finished ophthalmic lens blank, the method comprising the steps of: - determining a closed curve representative of a geometry of a gasket to be used in combination with the mould with the closed curve being near the periphery of the mould, wherein the determined closed curve corresponds to a desired shape of an edge of the mould, wherein the gasket to be used in combination with the mould bears on a facet of the mould along a closed curve, which closed curve is defined outside of the predetermined closed curve on the surface of the mould ; and - using an optimization process with desired optical properties of the semi-finished ophthalmic lens blank to be cast in the mould as inputs of the optimization process and the determined closed curve as constraints of the optimization process, to design a surface of the mould , so that said surface of the mould , which is regressive and comprises the facet extending outwardly from said edge, substantially fits the determined closed curve with a distance between each point of the determined closed curve and said surface of the mould being less than 0.1 mm, wherein said determined closed curve is comprised in one plane and the facet is comprised in said plane, and - providing the mould - providing the gasket , the predetermined closed curve used for designing the mould being representative of said gasket , - assembling the mould and the gasket . - determining a closed curve representative of a geometry of a gasket to be used in combination with the mould with the closed curve being near the periphery of the mould, wherein the determined closed curve corresponds to a desired shape of an edge of the mould, wherein the gasket to be used in combination with the mould bears on a facet of the mould along a closed curve, which closed curve is defined outside of the predetermined closed curve on the surface of the mould ; and - using an optimization process with desired optical properties of the semi-finished ophthalmic lens blank to be cast in the mould as inputs of the optimization process and the determined closed curve as constraints of the optimization process, to design a surface of the mould , so that said surface of the mould , which is regressive and comprises the facet extending outwardly from said edge, substantially fits the determined closed curve with a distance between each point of the determined closed curve and said surface of the mould being less than 0.1 mm, wherein said determined closed curve is comprised in one plane and the facet is comprised in said plane, and so that said surface of the mould , which is regressive and comprises the facet extending outwardly from said edge, substantially fits the determined closed curve with a distance between each point of the determined closed curve and said surface of the mould being less than 0.1 mm, wherein said determined closed curve is comprised in one plane and the facet is comprised in said plane, and - providing the mould - providing the gasket , the predetermined closed curve used for designing the mould being representative of said gasket , - assembling the mould and the gasket . <EOS>