CAPT'S ALTIMETER (COUPLED TO THE AUTOPLT) NOT SET IN TRANSITION THROUGH FL180 TO 17000 FT LEADING TO ALTDEV.

Narrative

I HAD TAKEN THE LAST BREAK ON AN ATLANTIC WBOUND XING AS PF (WHICH IS NOT THE BEST SIT; HOWEVER; THE CAPT HAD TAKEN THE MIDDLE BREAK); SO I HAD JUST COME BACK TO THE COCKPIT AS IT WAS TIME TO START THE DSCNT. I WAS TRYING TO GET UP TO SPD AS A FLT ATTENDANT CAME UP TO DISCUSS A CABIN PROB. I TOLD HER I COULD NOT TALK ANY MORE ABOUT IT AT THAT TIME; I WAS TOO BUSY. AT THAT TIME; WE WERE GIVEN A DSCNT FROM FL200 TO 17000 FT. I HAD NOT LOOKED AT THE ATIS THE CAPT HAD WRITTEN DOWN OR HEARD THE ALTIMETER SETTING FROM THE CTLR DUE TO THE CONVERSATION WITH THE FLT ATTENDANT. GOING THROUGH FL180 I LOOKED TO THE STANDBY ALTIMETER FOR THE SETTING AND THE CAPT HAD NOT SET IT THERE AS IS CUSTOMARY EITHER. BY THE TIME I LOOKED AT THE ATIS THE CTLR REPEATED THE LCL ALTIMETER AS 29.17 (ATIS SHOWED 29.30). THE AUTOPLT WAS FLYING WITH AN APPROX 2000 FPM DSCNT RATE. AS SOON AS I STARTED SPINNING THE ALTIMETER SETTING IN MY SIDE (THE CAPT HAD YET TO SET EITHER OF HIS) I REALIZED THE AUTOPLT WOULD CAPTURE BASED ON THE CAPT'S ALTIMETER (GREATER THAN 700 FT TOO LOW). I DISCONNECTED THE AUTOPLT AND LEVELED OFF BASED ON MY ALTIMETER AS GENTLY AS I COULD WITHOUT UPSETTING THE CABIN. THIS TOOK ME TO 16750 FT MSL (250 FT LOW). I ESTABLISHED 17000 FT LEVEL AND WHEN THE CAPT HAD SET HIS ALTIMETERS I REENGAGED THE AUTOPLT AND TRIED TO REGAIN MY COMPOSURE (NOT EASY; I PRIDE MYSELF ON THINKING AHEAD OF THE AIRPLANE AND CAPT). SEVERAL THINGS COULD HAVE AVOIDED THIS PROB: 1) NOT ALLOWING THE FLT ATTENDANT TO DISCUSS ANYTHING AFTER THE TOP OF DSCNT. 2) AN AUTOPLT THAT FOLLOWS THE AIM OF 1000 FPM RATE IN THE LAST 1000 FT OF DSCNT. 3) THE CAPT COULD HAVE BROUGHT THE LOW ALTIMETER SETTING TO MY ATTN AFTER LISTENING TO THE ATIS. 4) START LOOKING FOR ALTIMETER SETTING LEAVING FL200 INSTEAD OF FL180. 5) EVEN CHANGING MODE ON AUTOPLT TO VERT SPD -- 1000 FPM -- DURING LAST 1000 FT OF DSCNT.