A CPR FLC OF AN LTT ENTERED THE WRONG COORDINATES FOR A NAV POINT IN THE FMS AND DUE TO THIS STARTED A TURN TOO EARLY.

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WHILE ENRTE TO HOU FROM SHV; OUR CLRNC WAS CHANGED FROM DAS1 STAR TO ROKET ARR. THE FMS HAS DATABASE OF VORS AND ARPTS; BUT INTXNS MUST BE MANUALLY ENTERED USING LATITUDE/LONGITUDE. THE ORIGINALLY FILED AND CLRED STAR WAS ENTERED PRIOR TO DEP. THE PNF BEGAN ENTERING THE REVISED CLRNC WHILE I FLEW THE DSCNT. IT WAS A VERY BUSY PHASE OF FLT AND I WAS UNABLE TO VERIFY THE LATITUDE/LONGITUDE OF ALL THE WAYPOINTS. I DID BACK UP THE COURSE TO BE FLOWN WITH VORS; BUT DIDN'T HAVE THE STAR IN FRONT OF ME TO VERIFY DISTANCES BTWN FIXES. THE PNF HAD TRANSPOSED THE MINS OF THE LATITUDE FOR THE FINAL FIX (DRXLR INTXN) WHERE WE WERE TOLD TO DEPART HDG 300 DEGS. HE ENTERED '54' INSTEAD OF '45.' THIS CAUSED US TO INITIATE THE TURN APPROX 10 NM EARLY. THE RERTE HAPPENED AT A VERY BUSY PHASE OF FLT; AS DID THE TURN AT DRXLR. ATC QUERIED US AND WE RESPONDED THAT WE WERE INITIATING THE TURN TO 300 DEGS AS INSTRUCTED AT DRXLR. HE RESPONDED THAT WE WERE ABOUT 10 MI OFF COURSE AND ISSUED US A TURN BACK ON COURSE. THE PNF IMMEDIATELY RECHKED THE LATITUDE/LONGITUDE AND FOUND THE ERROR. WE CHASTISED OURSELVES FOR NOT ALWAYS HAVING THE PF BACK UP THE PNF'S ENTRIES. HOWEVER; AS PF; I WAS QUITE BUSY FOLLOWING THE DSCNT PROFILE AND SPD XING RESTRS AS WELL AS ACCOMPLISHING THE CHKLISTS FOR DSCNT; TRANSITION LEVEL; AND APCH; AND TRYING TO HELP WITH RADIO TUNING. THE PNF ALSO HAD HIS PART IN ALL THESE ACTIONS ALONG WITH RADIO CALLS TO DISTRACT HIM. IN RETROSPECT; I SHOULD HAVE HAD THE STAR IN FRONT OF ME TO BACK UP THE DISTANCES IN ADDITION TO THE RADIALS FOR VOR BACKUP.