A B757-200 WAS DISPATCHED IN OVERWATER CONFIGN WITH THE CTR TANK FUEL QUANTITY INDICATOR INOP IN CONFLICT WITH THE MEL.

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BASE INFORMED ME THAT THE CTR FUEL QUANTITY INDICATOR WAS BLANK. I CHKED HISTORY ON THE ACFT AND NOTICED THAT IT HAD PREVIOUS WRITE-UPS AND THAT IT HAD BEEN DEFERRED ON APR/XA/01 UNDER MEL ABC-1. THEREFORE; I LOOKED AT THE MEL REF; READ THE INFO; AND NOTIFIED BASE TO CALL DISPATCH AND OPS BASE. WITH PLACARD XXX; SOME FEW MINS LATER; BASE CALLED ME BACK THAT IT WAS APPROVED AND TO USE MR X. AT THAT TIME I EXPLAINED TO DISPATCH TO NOTIFY FUELERS TO DRIP STICK TANK AND THAT IT MUST REMAIN EMPTY. ABOUT 5 MINS LATER; BASE CALLED BACK THAT THE CREW HAD THE TOTAL FUEL QUANTITY INDICATOR BLANK. I EXPLAINED THAT IT WAS CAUSED BY THE CTR FUEL QUANTITY INDICATOR BEING BLANK; AND JUST TO BE SURE ON MY PART I CALLED COMPANY LINE MGR AND EXPLAINED OF THE CTR TANK BEING BLANK AND THAT I HAD DEFERRED IT AND THAT THE TOTALIZER WAS BLANKING ALSO. HE SUGGESTED ME DEFERRING THE TOTALIZER ALSO; TO BE ON THE SAFE SIDE. AND SO I DID. MEL ABC-1 HDG SAYS 'FUEL QUANTITY INDICATION SYS (FLT DECK) CTR TANK INDICATOR -- TANK REMAINS EMPTY.' TO ME; IT APPLIES TO ALL B757 ACFT. NOTHING IS SAID ABOUT ETOPS OR OVERWATER ACFT. SPECIAL NOTES; AND OPS PLACARD DOES NOT MENTION ANYTHING ABOUT ETOPS OR OVERWATER. BUT IN MANY AREAS IT SAYS THAT CTR FUEL TANK MUST REMAIN EMPTY. THE CREW KNOWS WHERE THEY ARE GOING AND IF THEY NEED FUEL IN CTR TANK. DISPATCH A LOT OF TIMES REFUSES AN MEL REF SAYING THAT WITH THAT PLACARD IT HAS A FUEL RESTR AND WOULD NOT MAKE IT TO ITS DEST. THE TRUTH OF THE MATTER IS THAT THE MEL IS VERY VAGUE IN A LOT OF AREAS. WE CONSTANTLY HAVE A HARD TIME TRYING TO FIGURE OUT WHAT MEL REF BEST APPLIES TO THE PROB WE ARE DEALING WITH. IN REALITY; THE MEL HAS A LOT OF GRAY AREAS THAT NEED TO BE CLARIFIED. FOR INSTANCE; FOR THIS CTR TANK FUEL INDICATION PROB; HAVE THE HDG SAY 'NON ETOPS ACFT ONLY' OR 'DOMESTIC ONLY.'