ALTDEV ALT OVERSHOT IN DSCNT.

Narrative

ATL CTR CLRED ME TO 6000 FT AND SWITCHED ME TO ATL APCH. I GAVE MY CALL SIGN AND SAID DSNDING TO 6000. ABOUT 6500 AND IN 500 FPM DSCNT; A PAX ASKED IF I WOULD CALL AHEAD TO FBO TO CONFIRM RENT-A-CAR WOULD BE WAITING. IT TOOK A WHILE TO FIND ARINC FREQ AND I DID NOT NOTICE PASSING 6000 FT. ABOUT 5200 ATL APCH CALLED TO TELL ME I WAS ASSIGNED 6000. IT DIDN'T SEEM (AT THE TIME) ODD TO BE LEVELING AT 5000. I WAS SURE I HAD BEEN CLRED TO 5000. I THINK IN MY MIND THE PROCESS OF LOOKING UP THE ARINC I HAD ARBITRARILY ASSUMED I COULD GET THE INFO WRITTEN THE MIN I HAD LEFT. WHEN I NEXT LOOKED UP TO THE ALTIMETER AND RECEIVED ATL APCH'S QUERY IT SEEMED LOGICAL TO BE LEVELING AT 5000. FURTHER CONVERSATION WITH ATL: APCH ASKED IF CTR HAD ASSIGNED ME 5000 AND I SAID YES. TO PROVE THIS TO MYSELF I LOOKED DOWN TO THE ALT ALERTER AND WAS DISMAYED TO SEE 6000. I THEN REALIZED MY ERROR AND ADMITTED TO ATL APCH 'IT MAY BE MY MISTAKE.' I ASKED IF HE WANTED ME TO CLB TO 6000. HE SAID NO; THAT 5000 WAS OK AND CLRED ME DIRECT TO THE ARPT. COMPOUNDING MY NOT SEEING THE ALT ALERTER WAS A NEWLY INSTALLED CUP-HOLDER MAKING IT DIFFICULT TO READ. I DO NOT RECALL HEARING THE BEEP NORMALLY ASSOCIATED WITH THE ALERTER EITHER. I HAVE REMOVED THE CUP-HOLDER. IN CONVERSATION WITH THE CHIEF PLT ABOUT THIS SITUATION; WE HAVE DECIDED TO MINIMIZE ATTEMPTS AT CHKING FOR RENT-A-CARS ETC; ESPECIALLY IN A BUSY ENVIRONMENT.