ALT ASSIGNMENT OVERSHOT BY A B737 FLT CREW WHILE CHANGING PLANS ON A HOLDING FIX ON FREQ WITH ZBW; MA.

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DSNDING INTO MHT AND PROCEEDING DIRECT TO EEN WE WERE GIVEN CLRNC FROM FL410 TO FL290. SHORTLY THEREAFTER; WE WERE TOLD THAT MHT HAD GONE BELOW LNDG MINIMUMS AND TO EXPECT HOLDING INSTRUCTIONS. WE ASKED IF OUR HOLDING INSTRUCTIONS COULD BE AT AS HIGH AN ALT AS POSSIBLE IN ORDER TO CONSERVE FUEL. WE WERE THEN CLRED DIRECT TO CON (CONCORD); WHICH WAS NOT ON OUR FLT PLAN AND TOLD TO HOLD AS PUBLISHED. I WAS THE PF AND NOT KNOWING EXACTLY WHERE CON WAS; I BEGAN LOOKING FOR IT ON HIGH ALT ENRTE CHART AFTER PUTTING IT INTO THE CDU. I COULDN'T FIND IT THERE AND THERE ARE NO STARS INTO MHT SO I BEGAN LOOKING; RUMMAGING THROUGH MY FLT BAG TO SEE IF THE PUBLISHED PATTERN WAS ON THE LOW CHART. ABOUT THIS TIME THE FO ANNOUNCED HE HAD FOUND THE HOLDING PATTERN ON THE LOW CHART AND THAT THE PATTERN IN THE CDU WAS INDEED VALID. A FEW SECONDS LATER ATC CALLED AND SAID; 'LOOKS LIKE YOU HAVE MISSED FL350; DSND TO FL330 AND WE'LL HAVE YOU HOLD THERE.' APPARENTLY; STOPPING THE DSCNT AT FL350 WAS PART OF THE HOLDING CLRNC ISSUED AND I SIMPLY MISSED IT. THE FO NEVER SET THE MCP FOR FL350 AS HE TOO WAS TRYING TO FIND THIS HOLDING PATTERN ON A CHART SOMEWHERE. ATC SAID THERE WAS REALLY NO HARM DONE AS WE HAD BEEN PREVIOUSLY CLRED TO FL290 ANYWAY; BUT I FEEL THAT THE SIT DESERVES MENTIONING HERE. CONTRIBUTING FACTORS WERE THE FOLLOWING: I DID NOT EXPECT TO HOLD AT FL350 15 MI FROM THE DEST ARPT EVEN THOUGH WE HAD ASKED FOR A HIGH ALT. ALTHOUGH THIS HELPED CONSERVE FUEL FOR OUR LATER DIVERSION; I HAD IN MIND A TEENS OR TWENTIES ALT. BEING TOLD TO HOLD AT A FIX THAT WAS NOT ON OUR RTE OF FLT AND NOT ON ANY CHART WE WERE LIKELY TO HAVE OUT INCREASED THE WORKLOAD ENORMOUSLY DURING A VERY BUSY STAGE OF FLT. A HOLDING PATTERN OVER EEN WOULD HAVE BEEN MUCH EASIER TO HANDLE. ALSO; I PROBABLY SHOULD HAVE SIMPLY LET THE FO FIND THE HOLDING PATTERN WHILE I FLEW. HOWEVER; ALT CHANGE TO FL350 NEVER REGISTERED WITH ME. (I STILL CAN'T SAY IF IT WAS ACTUALLY ISSUED.) I THOUGHT I HAD PLENTY OF TIME TO MONITOR A LEVELOFF AT FL290 SO I DON'T THINK THIS WOULD HAVE HELPED UNDER THE CIRCUMSTANCES.