AN MD80 CAPT RPTED THAT HE DEPARTED YUL WITHOUT THE WX BEING RPTED IN MI; AS IS REQUIRED BY THE FARS TO BE LEGAL. HOWEVER; THE MEASUREMENT; GIVEN IN FT; INDICATED THERE WAS SUFFICIENT VISIBILITY FOR DEP.

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FOGGY CONDITIONS AT YUL ARPT. WE EXPECTED RWY 24L FOR TKOF AND REVIEWED TKOF LEGALITY FOR RWY 24L/RVR 600 FT. PREVAILING VISIBILITY WAS RPTED AT 1/8 MI. ENRTE TO RWY 24L; THE DEP RWY WAS CHANGED TO RWY 6R. VISIBILITY REQUIRED FOR RWY 6R WAS 1/4 MI. WE WERE THIRD IN LINE FOR TKOF AND THE VISIBILITY STARTED IMPROVING RAPIDLY. TWR RPTED RVR AT 3300 FT. THE 2 ACFT AHEAD DEPARTED. RVR-A WAS RPTED AGAIN AT 3300 FT. I MENTALLY CALCULATED THAT TO BE WELL ABOVE 1/4 MI AND WE TOOK OFF. DURING LATER DISCUSSION; WE DETERMINED THAT WE SHOULD HAVE REQUESTED A VISIBILITY RPTED IN MI AS THAT WAS THE CTLING FACTOR FOR RWY 6R AND THE RVR VALUES BEING CALLED BY THE TWR; TECHNICALLY PERTAINED TO RWY 24L ONLY. THIS ATIS PUBLISHED 5 MINS AFTER OUR TKOF RPTED VISIBILITY AT 1/2 MI. I AM CONFIDENT IN THE SAFETY OF MY DECISION TO DEPART; BUT SHOULD HAVE ASKED FOR THE VISIBILITY IN MI; AS REQUIRED BY FARS; TO BE LEGAL. I DON'T THINK THE TWR SHOULD VOLUNTEER RVR VALUES IF THEY ARE NOT PERTINENT TO THE RWY IN USE. THIS ACTION JUST SETS A TKOF LEGALITY TRAP FOR THE PLTS.