LGT FLC FAILS TO RESET ALTIMETER DSNDING THROUGH FL180. ALTDEV.

Narrative

FO WAS PF. WE RECEIVED A CLRNC TO CROSS 40 DME SW ARMEL AT 11000 FT. ATIS WAS OVER 1 HR OLD AND WE HAD SOME DISCUSSION AS TO WHAT APCH WE WOULD FLY. WE BRIEFED THE IAD ILS TO RWY 19R AND THEN THE CAT II TO RWY 1R. THE FO STARTED HIS DSCNT LATE. PASSING FL190 HE REALIZED WE WOULDN'T MEET THE XING RESTR UNLESS HE USED SPD BRAKES. AT THIS POINT THE FO GOT MORE AGGRESSIVE WITH HIS DSCNT. PASSING FL180 I CALLED FOR THE IN-RANGE CHKLIST AND SET MY ALTIMETER TO THE CURRENT SETTING OF 29.63 INCHES. AT THIS POINT WE GOT A FREQ CHANGE TO IAD APCH CTL AND RECEIVED A RWY ASSIGNMENT TO THE ILS RWY 12. DUE TO THE DISTRACTIONS OF 3 DIFFERENT APCH BRIEFS PLUS THE LATE DSCNT THE FO FAILED TO SET HIS ALTIMETER TO 29.63 INCHES; LEAVING IT AT 29.92 INCHES. THE SO FAILED TO VERIFY THE CORRECT SETTING DURING THE IN-RANGE CHK. THE FO WAS USING HIS ALTIMETER DURING THIS AGGRESSIVE DSCNT SO HE LEVELED OFF TO MEET THE XING RESTR AT 40 DME; BUT ENDED UP 300 FT LOW WITH REF TO THE CAPT (XPONDER) ALTIMETER (WHICH WAS CORRECTLY SET AT 29.63 INCHES). WE IMMEDIATELY CLBED BACK TO 11000 FT AS ASSIGNED. I AM NOT AWARE OF ANY TFC CONFLICTS.