The present invention relates to a method and to an instrument for determining the limiting margin of a surveillance parameter of a turboshaft engine. During a preliminary stage, a secondary processor means (11) determines a preliminary comfort margin. Thereafter, during a main stage, it estimates a useful comfort margin from the preliminary comfort margin, and then an apparent difference between the current value and the limit value of the surveillance parameter, and finally the limiting margin by subtracting the useful comfort margin from the apparent difference.