CAPT OF A B757 OVERSHOT DSCNT ASSIGNED ALT DUE TO A MISUNDERSTANDING OF THE CORRECT ALT. HOWEVER; AFTER FO REQUESTED CLARIFICATION FROM ATC; THEY WERE RECLRED TO STAY AT THAT ALT.

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ON DSCNT INTO MEM PASSING 9200 FT FOR 8000 FT ON THE GILMORE STAR; APCH TOLD US TO LEVEL AT 9000 FT AND MAKE A L 360 DEG TURN TO REJOIN THE ARR. I ACKNOWLEDGED THE NEW CLRNC AND OBSERVED THE CAPT SLEWING THE HDG TO THE L ON THE MCP. I THOUGHT I SAW HIM REACH FOR THE ALT KNOB AND AT THAT TIME WENT HEAD DOWN TO REPROGRAM THE FMS TO INTERCEPT THE ARR UPON THE COMPLETION OF OUR TURN. AFTER ABOUT 90 DEGS OF HDG CHANGE; I LOOKED UP AND SAW THAT WE WERE LEVELING AT 8000 FT. I INFORMED THE CAPT THAT OUR ASSIGNED ALT WAS 9000 FT. I CALLED ATC TO VERIFY AND HE TOLD US TO JUST REMAIN AT 8000 FT NOW. REMAINDER OF FLT WAS UNEVENTFUL. COMPANY PROCS DICTATE THAT THE PF WILL MAKE ALL MCP CHANGES (INCLUDING ALT) WITH THE AUTOPLT ON. I SHOULD HAVE TECHNICALLY VIOLATED THIS POLICY AND SPUN IN 9000 FT. SINCE WE WERE IN A HIGH WORKLOAD; SHORT FUSE SCENARIO; I ALSO COULD HAVE BEEN MORE POSITIVE ABOUT VERIFYING THAT THE CAPT HAD HEARD THE ENTIRE CLRNC (LEVEL AT 9000 FT) AND NOT JUST THE PART ABOUT MAKING THE L 360 DEG TURN. BOTTOM LINE: SHOULD HAVE TO PUT THE RIGHT ALT IN THE WINDOW.