MD80 FLT CREW DISCOVERS A FUEL DISCREPANCY DURING CRUISE FLT.

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HERE'S THE PROB: I'VE BEEN ON THE MD80 FOR OVER 14 YRS NOW; AND AS FAR AS I CAN REMEMBER; THIS WAS THE FIRST TIME I ACTUALLY HAD BALLAST FUEL TO DEAL WITH. UNFORTUNATELY; THIS LACK OF EXPERIENCE LED ME DOWN AN INSIDIOUS PATH. THE ACFT HAD BEEN FUELED TO 19300 LBS. THE FLT PLAN HAS 19218 LBS AS THE RELEASE FUEL. THE LOAD PAPERWORK ALSO HAD 19200 LBS OF FUEL LISTED. THE FO AND I AGREED TO LEAVE THE 5000 LBS OF FUEL IN THE CTR TANK ALONE AND IT WAS NOT USED ON THE FLT. I DO NOT KNOW WHY; BUT I SOMEHOW MISREAD THE FUEL ON THE LOAD CLOSEOUT. I COULD HAVE SWORN IT MATCHED THE OTHER QUANTITIES I MENTIONED. I DID NOTE WE HAD TUL AS AN ALTERNATE. I THOUGHT THIS WAS UNNECESSARY AS THE WX IN STL WOULD REMAIN VMC WELL AFTER OUR ARR. DISPATCH GAVE US 5000 LBS FOR THE ALTERNATE FUEL. SHORTLY AFTER LEVELING OFF AT FL230; THE FO VOICED HIS CONCERN REGARDING THE DISPARITY BTWN THE LOAD CLOSEOUT FUEL AND WHAT WAS ON THE FLT PLAN. I SOON REALIZED THE FUELER MUST HAVE INCORPORATED THE BALLAST FUEL INTO HIS FUELING OF THE ACFT WITH THE RESULT OF US HAVING 5000 LBS LESS USABLE FUEL ON BOARD. WE QUICKLY EVALUATED THE SITUATION AND CAME TO THE CONCLUSION WE WOULD HAVE ENOUGH FUEL TO GET TO STL WITHOUT PROB. THE QUESTION WE BOTH HAD BURNING IN OUR MINDS WAS: WHAT ABOUT THE CTR OF GRAVITY? I GOT A HOLD OF DISPATCH; ADVISING HIM OF OUR SITUATION. AFTER CHKING WITH LOAD CTL; WE WERE RECLRED WITH 18800 LBS OF FUEL AND TUL WAS DROPPED AS AN ALTERNATE. THE REST OF THE FLT WENT WITHOUT INCIDENT. I AM TO BLAME FOR NOT CAREFULLY CHKING THE LOAD CLOSEOUT. HAD I DONE A BETTER JOB; WE WOULD HAVE DISCOVERED THIS DISCREPANCY AND CORRECTED THE SITUATION BEFORE WE EVEN LEFT THE RAMP.