ACR FLT DEPARTS IN IMC WITHOUT LEGALLY RPTED TKOF MINIMUMS AS REQUIRED TO BE RPTED BY ATC.

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AT THE SCHEDULED TIME OF OUR DEP FROM MCALLEN; TX; THE CURRENT WX RPT (ATIS) INDICATED A VISIBILITY OF 1/8 SM IN FOG. TAXIING OUT FOR RWY 13 WE FOUND CONDITIONS WERE VARIABLE BUT WERE BELOW THE 1/4 MI NECESSARY FOR DEP. AFTER ASSESSING THE SIT I ELECTED TO HOLD IN THE RUN-UP AREA UNTIL VISIBILITY IMPROVED. AFTER WAITING FOR A LITTLE OVER 1 HR WE NOTED THAT CONDITIONS HAD IMPROVED. THE SUN WAS NOW UP AND THE SURFACE WIND HAD PICKED UP TO 5 KTS. ALSO; A CESSNA LANDED OUT OF AN ILS APCH AND RPTED PICKING UP THE RWY AT DECISION HT AND THAT VISIBILITY DOWN THE RWY WAS 'GOOD.' FROM OUR POS I WAS LOOKING ACROSS THE TOUCHDOWN ZONE; COUNTING RWY EDGE LIGHTS. WHEN WE PICKED OUT THE 7TH LIGHT I ESTIMATED THE VISIBILITY AT BETTER THAN 1400 FT. TAXIING INTO POS ON RWY 13 BOTH THE FO AND I COULD SEE PAST THE 8TH EDGE LIGHT. AT 200 FT BTWN LIGHTS THIS WORKED OUT TO MORE THAN 1600 FT OR 1/4 MI; THE MINIMUMS ALLOWED FOR DEP. I ADVISED THE TWR OF THE VISIBILITY AND REQUESTED A TKOF CLRNC. MCALLEN DOES NOT HAVE RVR; NO RVV WAS GIVEN. THE HIGH INTENSITY RWY LIGHTS WERE WORKING AND THE RWY CTRLINE MARKINGS WERE CLRLY VISIBLE. DURING THE TKOF ROLL WE HAD ADEQUATE FORWARD VISIBILITY FOR A SAFE DEP. THE PROB HERE IS THAT I CONFUSED THE REQUIREMENTS OF FAR 121.651; PARAGRAPH (A) WITH THE PROVISIONS OF PARAGRAPH (B). TKOF MINIMUMS ARE BASED ON RPTED VISIBILITY; AN APCH AND LNDG CAN BE COMPLETED BASED ON THE PLT'S ASSESSMENT OF ACTUAL CONDITIONS IF THE RPTED WX GOES BELOW MINIMUMS AFTER THE ACFT HAS BEGUN THE FINAL APCH SEGMENT OF AN INST APCH PROC. (REF 121.651(C).) A QUICK CHK OF OUR COMPANY'S FLT OPS MANUAL PRIOR TO THE MFE DEP DID NOT DISPEL MY CONFUSION ON THIS ISSUE. REREADING THE FAR'S HAS CLRLY SHOWN THAT I WAS IN ERROR. MY MISTAKE WAS NOT INTENTIONAL BUT ONE OF MISUNDERSTANDING.