ImmixFilms

I'm a film student and at my school we've been getting conflicting reports on how to measure for depth of field when shooting a subject's reflection in a mirror. Do you measure from your camera to the mirror's surface, or Do you measure to the mirror and then from the mirror to the subject to find the total distance? And why? I'm not the only one at my school that is questioning this, so your answer would help out a lot of future filmmakers. Thanx.