ATR42 CAPT READJUSTED THE APCH AND LNDG SPD AFTER NOTICING THAT THE FORWARD CARGO COMPARTMENT HAD ONLY A 6TH OF THE WT INDICATED IN THE FLT MANIFEST. A SLOWER SPD DUE TO LESS ACFT WT WAS CRITICAL IN THEIR DEST LNDG DUE THE RELATIVE SHORT RWY LENGTH.

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AT LEVELOFF AT 15000 FT; WE RECEIVED A 'MASTER WARNING; DOOR UNLOCK LIGHT ON' INDICATING THAT THE REAR 'EMER SVC' DOOR WAS UNLOCKED. THE CAPT NOTIFIED THE FLT ATTENDANT AND ASKED HIM TO CHK THE DOOR GREEN TAB INDICATORS. THE FLT ATTENDANT STATED THAT THE TABS SHOWED THE DOOR TO BE LOCKED AND IN A SAFE CONDITION. WE COMPLETED THE REST OF THE CHKLIST AND ELECTED TO CONTINUE TO OUR DEST SINCE THE ACFT WAS NOT INDICATING ANY PRESSURIZATION PROBS AND THE FLT ATTENDANT DID NOT NOTICE ANY SOUNDS LIKE LEAKING AIR. THE CAPT DECIDED TO GO TO THE BACK OF THE ACFT AND TAKE A LOOK ANYWAY. WHEN HE EXITED THE SMOKE DOORS AND ENTERED THE FORWARD CARGO AREA DIRECTLY BEHIND THE COCKPIT; HE CALLED TO ME TO LOOK BACK. I DID; AND HE SAID 'THERE'S NOT 600 LBS OF CARGO IN #1 AND THE NET IS NOT SECURE!' HE PROCEEDED BACK TO CHK THE DOOR WHICH WAS OK. WHEN HE RETURNED; WE RECHKED THE WT AND CTR OF GRAVITY AND FOUND THAT WHILE NOT OUT OF LIMITS IT WAS VERY CLOSE. THE WTS WERE AN ISSUE BECAUSE WE WERE TRAVELING TO AXA; OUR SHORTEST STRIP AND A FLAPS 45 DEG APCH. IF WE HAD NOT DISCOVERED THE ERROR IN WT; WE WOULD HAVE CALCULATED APCH SPDS MUCH HIGHER THAN NEEDED AND COULD HAVE HAD A PROB LNDG AT THIS 3600 FT STRIP. I GUESS WE WERE LUCKY THE DOOR LIGHT ILLUMINATED OR WE WOULD NOT HAVE DISCOVERED THE ERROR. THE DOOR LIGHT EXTINGUISHED ITSELF SHORTLY AFTER WE WORKED OUT THE NEW AND CORRECT SPDS. ANGELS? CARGO 1 HAD ONLY 90 LBS.