The disclosed embodiments offer a solution for controlling the thickness of a finished structural composite part when precise tolerances must be complied with for the quality of the assembly and in order to ease the assembly operations of this part. In order to implement the method according to the disclosed embodiments, an additional layer of plies made out of a non-structural composite material, called calibrating plies, is applied onto the structural plies of the structural part, at least in an area where the thickness must be maintained within close tolerances and, after the composite material has been cured, the calibrating plies are machined in order to obtain the thickness required for the finished part. The thickness of the calibrating plies applied is determined according to the possible thicknesses of structural plies, taking into account the manufacturing spreads so that the machining operation for the calibrating plies does not affect the structural plies.