AN ACR FLC OVERSHOT THEIR ASSIGNED ALT DURING THE DSCNT. THE ALTIMETERS HAD BEEN SET CORRECTLY; BUT THE CONFUSION THAT AROSE ABOUT THE CORRECT SETTING; AT LESS THAN 1000 FT ABOVE THE LEVEL OFF ALT; LED TO AN OVERSHOOT.

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DURING DSCNT INTO PHL (FLT WAS FROM DFW) ON THE DUPONT 3 ARR. CLRNC WAS TO CROSS JAYBO INTXN AT 15000 FT MSL. AT 18000 FT; TRANSITION ALT; ALL 3 ALTIMETERS (CAPT; FO'S AND MIDDLE ALTIMETER) WERE SET TO 29.40 INCHES. AS THE CAPT WAS SETTING THE ALTIMETER HE SAID '39?' AND I SAID 'NO; 40; 29.40 INCHES.' THE ACFT WAS HOOKED UP TO THE AUTOPLT AND SOMEWHERE BTWN 16000 FT AND 15000 FT (1000 FT ABOVE LEVEL OFF) THE CAPT BEGAN SPINNING THE CTR ALTIMETER BAROMETRIC KNOB FIRST ONE WAY AND THEN THE OTHER. I WAS FLYING THE ACFT AND WITH THE CTR ALTIMETER AS MY MAIN INST FOR XCHK FOR ALT LEVEL OFF; THE AUTOPLT WENT THROUGH 15000 FT DOWN TO 14600 FT WHEN THE CAPT HAD SPUN THE BAROMETRIC KNOB UP AND DOWN IN CONFUSION. THE BAROMETRIC ALTIMETER SETTINGS WERE CORRECT AT 18000 FT; SET FOR LCL 29.40 INCHES; BUT SOMEWHERE DURING DSCNT FROM THE TRANSITION ALT TO PASSING 16000 FT AND ON DOWN TO 15000 FT; THE CAPT WAS DISTRACTED AND HAD RESET THE CTR ALTIMETER TO 30.40 INCHES (THIS ALSO HAPPENED TO BE WHAT OUR ALTIMETER SETTING WAS WHEN WE LEFT DFW). PERHAPS COCKPIT DISCUSSIONS WERE DISTRACTING ENOUGH TO CAUSE THE CONFUSION BTWN SETTINGS 29.40 INCHES AND 30.40 INCHES. ALSO; THE CAPT SEEMED CONCERNED THROUGHOUT THE FLT THAT HE WAS COMING BACK ON HIS FIRST TRIP AFTER 9 WKS ABSENCE DUE TO BOTH VACATION AND SICK LEAVE. AT AN ACTUAL ALT OF 14600 FT MSL THE PHL CTLR BROUGHT ACTUAL ALT TO OUR ATTN AND THE ACFT WAS CLBED TO IMMEDIATE LEVEL OFF AT 15000 FT. I THINK THAT SOMETIMES THE ALTIMETER PROCS WE HAVE AT OUR AIRLINE (WHICH HAS THE CAPT AND THE FO SETTING A CONVERSION ALTIMETER BELOW 10000 FT TO QFE AND THE MIDDLE ONE TO QNH) CAN BE ADDITIONALLY CONFUSING IN AN ALREADY DISTRACTING AND COMPLEX FLYING ENVIRONMENT. PERHAPS A PERSONAL PRACTICE OF STERILE COCKPIT BELOW 18000 FT INSTEAD OF BELOW 10000 FT IS ALSO ADVISABLE.