ALTIMETER SETTING RULE NOT FOLLOWED IN DSCNT PROC OUT OF FL180. ALTDEV ALT OVERSHOT IN DSCNT.

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BUSY AND SOMEWHAT FATIGUED; WE WERE DSNDING INTO PDX FOR APCH DEALING WITH ICING AND AVOIDING AREAS OF HEAVIER PRECIPITATION ON OUR WX RADAR. WE HAD BEEN IN THE CLOUDS FROM MID TWENTIES ALT WISE. PRIOR TO THAT WE WERE BRIEFING APCH AND READYING THE ACFT FOR DSCNT THROUGH ICING. THINKING FAR ENOUGH AHEAD WE RAN ALL CHKLISTS. I THINK UPON REACHING 18000 FT; BUSY WITH ICING AND WX; WE OVERLOOKED RUNNING THE 18000 FT ITEMS OF THE CHKLIST AND NEITHER OF US CAUGHT THE FACT THE ALTIMETERS HAD NOT BEEN SET TO I BELIEVE 29.54. ONLY AFTER THE CTLR CALLED US ASKING IF WE WERE LEVEL AT 5000 FT AND HE WAS SHOWING 4500 FT DID I IMMEDIATELY INITIATE A CLB AND RESET THE ALTIMETER. I COMPLIMENT THE CTLR AND HAVE BECOME MUCH MORE CRITICAL OF MYSELF REGARDING 18000 FT AWARENESS; WHICH I THOUGHT WAS SUFFICIENT AWARENESS. BECAUSE OF ITS CRITICALNESS; I HAVE INSTITUTED NOT ONLY 18000 FT CHK; BUT 2 MORE CHKS: ONE AT INITIAL ALT ABOVE OR BELOW 18000 FT; EXAMPLE: '16000 FT ON 29.54 INCHES' SET AND ON INITIAL VECTOR FOR APCH ON PROC TURN INBOUND; EXAMPLE: 5000 FT ON 29.54 SET. THE NATURE OF OUR BUSINESS IS THAT A SIMPLE LITTLE NUMBER IN A WINDOW CAN MEAN THE DIFFERENCE BTWN A PLEASANT RIDE AND DISASTER.