AIR TAXI CARGO PLT DSNDS BELOW ASSIGNED ALT. CTLR CATCHES THE ERROR.

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I WAS BEGINNING MY DSCNT FROM CRUISE ON A FLT FROM PHL TO AGC. IT WAS THE FIRST OF 6 LEGS ON MY PART 135 CARGO RUN. ZOB (POSSIBLY PITTSBURGH APCH; I'M NOT 100 PERCENT SURE) CLRED ME TO DSND AND MAINTAIN 5000 FT. I WAS CURRENTLY AT 6000 FT. AS I WAS DSNDING THROUGH 4600 FT ATC TOLD ME HE WAS SHOWING ME 400 FT LOW ON HIS MODE C. HE GAVE ME THE CURRENT ALTIMETER AND RESTATED TO MAINTAIN 5000 FT. I ACKNOWLEDGED MY MISTAKE; APOLOGIZED; AND PROMPTLY CLBED BACK TO 5000 FT. NOTHING FURTHER WAS SAID. HOW PROB AROSE: IN MY MIND I WAS THINKING THAT I WAS CLRED TO 3000 FT INSTEAD OF 5000 FT. IN FACT; I EVEN SET 3000 FT IN MY MAKESHIFT ALT ALERTER (A USED ADF MOVABLE CARD INDICATOR). CONTRIBUTING FACTORS: THERE WAS 1 MOMENT DURING MY MISTAKEN DSCNT THAT I QUESTIONED MYSELF ABOUT THE ALT IN MY MIND. I VERY NEARLY ASKED ATC TO CONFIRM MY ALT LIMIT BUT DECIDED NOT TO; DISMISSING MY UNSUREDNESS AS PARANOIA. HOW IT WAS DISCOVERED: IT WAS ONLY DISCOVERED AFTER ATC ALERTED ME TO THE FACT THAT I WAS 400 FT BELOW MY CLRED ALT. HAD HE NOT MADE THE COMMENT THEN I WOULD HAVE DSNDED TO 3000 FT. CORRECTIVE ACTIONS: AFTER REALIZING THE MISTAKE; I STOPPED MY DSCNT AND CLBED BACK TO 5000 FT. CONCLUSION: WHY I WAS THINKING 3000 FT INSTEAD OF 5000 FT IS STILL NOT CLR TO ME. MAYBE I WAS LOOKING AT MY APCH PLATE DURING MY APCH BRIEFING AND NOTED THE 3000 FT INITIAL APCH ALT ON THE ILS RWY 28 PROFILE. ONE THING IS CLR. THERE WAS A MOMENT WHEN THE LITTLE VOICE IN MY HEAD SPOKE OUT AND I IGNORED IT. IN THE FUTURE; TO PREVENT A RECURRENCE; I WILL NOT HESITATE TO ASK QUESTIONS AND/OR CONFIRM ANY DOUBTS I HAVE WHEN A SIMILAR EVENT OR SIT OCCURS.