A DSNDING B757-200 FLC OVERSHOOTS THEIR ASSIGNED ALT WHEN THE PIC; PNF; ENTERS THE WRONG ALT INTO THE FMC WITHOUT ASKING FOR VERIFICATION FROM THE FO NE OF YQO; ON.

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AT APPROX 50 NM NE OF YQO AT FL280; TORONTO CTR ISSUED A CLRNC TO CROSS 15 NW OF AYLMER AT FL230. AFTER THE CAPT READ BACK THE CLRNC; THE CTLR CORRECTED THE CLRNC TO CROSS 15 MI NE OF AYLMER AT FL230. THE CAPT BEGAN PROGRAMMING A FIX 15 MI NE OF AYLMER IN THE FMC; WHILE I SET 23000 FT IN THE ALT WINDOW OF THE MODE CTL PANEL AND SELECTED FLT LEVEL CHANGE. AS THE AIRPLANE BEGAN DSNDING; I ADJUSTED THRUST TO PLACE THE GREEN ARC FOR LEVELOFF 2/3 OF THE DISTANCE FROM OUR PRESENT POS TO YQO ON THE HSI MAP DISPLAY; WHILE WAITING FOR THE XING FIX THAT THE CAPT WAS GUIDING FOR 15 MI NE OF YQO; TO VERIFY LEVELOFF BEFORE REACHING THE XING RESTR. THE CAPT COMPLETED BUILDING THE FIX IN THE FMC; EXECUTED IT WITHOUT ASKING ME TO CHK HIS ENTRY; THEN PROCEEDED TO CALL THE COMPANY ON THE OTHER RADIO. I VERIFIED THAT THE GREEN ARC INDICATED LEVELOFF BEFORE THE XING RESTR; THEN NOTICED THE DATA LINE ASSOCIATED WITH THE FIX ON THE HSI MAP READ 15000 FT. BELIEVING I HAD MISUNDERSTOOD THE XING RESTR; I QUICKLY SET 15000 FT IN THE ALT WINDOW; AND EXTENDED SPD BRAKES AT APPROX FL250 WITH 15 MI TO THE FIX; AND TURNED TO ASK THE CAPT TO VERIFY THE ALT AT THE FIX; ASKING; 'IS THE XING RESTR AT 15 OR 23?' AND POINTING TO THE DATA LINE NEXT TO THE FIX ON THE HSI MAP SCREEN. THE CAPT SEEMED TO HESITATE; SO I SAID; 'WE BETTER CHK WITH THE CTLR.' AT THAT TIME THE CTLR ASKED WHAT ALT WE WERE DSNDING TO. WE WERE PASSING ABOUT FL220 AT THAT TIME. REALIZING THE ERROR; I HIT ALT HOLD ON THE MCP; RETRACTED SPD BRAKES; SET 23000 FT IN THE ALT WINDOW AND SELECTED FLT LEVEL CHANGE TO CLB BACK TO FL230. OUR ALT WENT BELOW FL220 AS THE AIRPLANE LEVELED THEN BEGAN TO CLB; WHILE THE CAPT TOLD THE CTR OF OUR MISUNDERSTANDING. TORONTO THEN CLRED US TO FL190 AND RESET THE ALT WINDOW TO 19000 FT AND RESELECTED FLT LEVEL CHANGE. THERE WAS NO TFC CONFLICT; AND WE CONTINUED TO DTW. CONTRIBUTING FACTORS: THE CAPT'S FMC ENTRY FOR THE XING FIXES WAS ENTERED AS: 'YQO/-15 15000 FT; INSTEAD OF: YQO/-15 23000 FT.' THE CAPT DID NOT ASK ME TO CHK HIS ENTRY BEFORE EXECUTING IT; AND I DID NOT NOTICE THE DIFFERENCE UNTIL THE FIX APPEARED ON MY HSI MAP SCREEN WITH THE DATA LINE NEXT TO THE FIX READING 15000 FT. AT THAT POINT I BELIEVED THAT I MAY HAVE MISSED AN AMENDED ALT IN THE CTLR'S CORRECTED CLRNC IN ADDITION TO THE CORRECTED DIRECTION (NE INSTEAD OF NW). ALTHOUGH WE WERE CLOSE TO THE FIX AT THIS TIME I BELIEVED WE COULD MAKE THE XING AT 15000 FT WITH AN INCREASED RATE OF DSCNT; YET STILL VERIFY THE ALT BEFORE DSNDING BELOW FL230; HOWEVER; WE WERE BELOW FL230 BEFORE THE ALT COULD BE VERIFIED. I SHOULD HAVE ASKED THE CTLR INSTEAD OF ASKING THE CAPT TO VERIFY; TO GET THE CORRECT INFO FASTER; STRAIGHT FROM THE SOURCE. THE CAPT WAS RELATIVELY NEW TO THE AIRPLANE; HAVING COMPLETED IOE JUST A COUPLE OF MONTHS EARLIER. THIS WAS HIS FIRST 'GLASS COCKPIT.' HE HAD TRANSITIONED FROM THE B727; RESULTING IN AN INCREASED WORKLOAD FROM HIS LAST ACFT DUE TO THE ABSENCE OF AN FE ON THE B757. THE EVENT OCCURRED AFTER THE 7TH HR OF OUR FLT WHICH DEPARTED OVER 2 HRS LATE DUE TO THE LATE ARR OF THE ACFT TO OUR DEP ARPT. IT WAS TOWARD THE END OF A NORTH ATLANTIC XING; SO SOME DEGREE OF FATIGUE MAY HAVE PLAYED A PART. RECENT FLTS HAVE HAD XING RESTRS REQUIRING HIGH RATES OF DSCNT; SO I MAY HAVE BEEN 'SPRING LOADED' TO DSND AGGRESSIVELY. IDEAS TO PREVENT A REOCCURRENCE: A DATA UPLINK; SO THAT WE COULD READ THE CLRNC AS WELL AS HEAR IT; USING AN ADDITIONAL SENSE (VISION) TO VERIFY TO PREVENT ERRORS. SIMPLIFY THE CLRNC; TO CROSS AT THE VOR RATHER THAN SOME SMALL DISTANCE FROM IT; OR SIMPLY TO DSND TO THE ALT WITHOUT SPECIFYING A FIX; USING 'EXPEDITE' IF NECESSARY. PUBLISH THE FIX AND ALT TO CROSS AS PART OF THE ARR PROC; SO IT CAN BE ANTICIPATED. SUPPLEMENTAL INFO FROM ACN 478813: WE REACHED APPROX 19300 FT. I BELIEVE THAT I SHOULD NOT HAVE ATTEMPTED TO WORK BOTH RADIOS AT ONCE; AND DO NOT INTEND TO EVER ATTEMPT IT AGAIN. ALSO OUR PROC OF BOTH PLTS POINTING AT ANY NEW ASSIGNED ALT PRIOR TO CLBING OR DSNDING WASDEVIATED FROM HERE.