The present invention provides a method of adjusting a hearing instrument that is at least partially insertable into an ear canal, the hearing instrument (1) comprising at least two microphones, the method comprising the steps of: estimating the relative microphone location effect for each of the microphones; estimating the feedback stability for each of the microphones; determining the optimum proportion and phase of the signals of the microphones to be used in an omni-directional mode; and setting the optimum proportion and phase of the signals of the microphones. Thus, the present invention takes into account the acoustical stability of each of the microphones in order to optimally combine the microphones to achieve an optimal omni-directional performance if desired by the user of the hearing instrument.