A B727 FLC EXPERIENCES AN ALTDEV ALT UNDERSHOT DURING DSCNT TO AN ASSIGNED ALT OF FL270.

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CTR GAVE A XING RESTR TO CROSS 80 DME S OF FAK AT FL270. AT THE TIME WE WERE CRUISING AT FL290 AT AROUND 120 DME S OF FAK WHEN INSTRUCTIONS WERE ISSUED. POLICY AT MY AIRLINE IS TO PUT THE ALT (FL270) IN THE ALERTER AND EACH PLT POINT AND CONFIRM. WE BOTH AGREED THAT FL270 WAS THE XING ALT; BUT THE CAPT THOUGHT HE HEARD 70 DME AND I WROTE DOWN 80 DME. I ALSO HAD THE AIRPLANE ON THE AUTOPLT; AND IT WAS HOLDING AN ALT AT CRUISE OF 28980 FT. WELL; I MISREAD THE ALTIMETER THINKING WE ONLY HAD 1000 FT TO LOSE WHEN ACTUALLY WE HAD 2000 FT. I JUST READ THE FIRST 2 DIGITS OF THE ALTIMETER. WELL; I MADE THE 1000 FT ALT CHANGE BY 80 DME AND AROUND 75 DME THE CAPT REMINDS ME OF THE ALT XING AND AT THAT POINT I REALIZE 2 THINGS: FIST; WE BOTH MISINTERPED THE CLRNC; AND SECOND THAT I HAD MISREAD THE ALTIMETER. WE WERE AT FL270 BY 70 DME HOWEVER I BELIEVED THAT WAS NOT THE CLRNC. I BELIEVE IT WAS FL270 BY 80 DME; HOWEVER THE CTLR (ZDC) DID NOT SAY ANYTHING. I BELIEVE THE CONTRIBUTING FACTORS TO BE: POSSIBLE FATIGUE (I HAD TO BE TAKEN OFF THE LAST LEG OF THE TRIP IN ORDER NOT TO EXCEED 30 HRS IN 7 DAYS); CREW FAMILIARITY POSSIBLY BREEDS LAZINESS; AND EVEN THOUGH IT'S NOT COMPANY REQUIREMENT I ALWAYS TRY TO VERBALLY CONFIRM XINGS WITH PNF -- SOMETHING I DID NOT DO IN THIS CASE; AND AT THE TIME ATC WAS ISSUING THE XING; A FLT ATTENDANT WAS IN THE COCKPIT WITH CREW MEALS.