MISCALCULATION OF SPACING BY LCL CTLR RESULTS IN UNCOMFORTABLE XING TFC BY B767 ON R BASE FOR RWY 35L WITH STRAIGHT IN ACFT FOR RWY 35R AT SDF.

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THIS EVENT OCCURRED EARLY ON A SUNDAY MORNING. WE WERE 1 OF ONLY A VERY FEW ACFT FLYING THE SKIES. ON APCH INTO SDF FROM THE E; WE WERE CLRED DIRECT TO THE FINAL APCH FIX FOR RWY 35L WHICH WOULD CARRY US ACROSS THE FINAL APCH PATH FOR RWY 35R. ON THE AUTO DEPENDENT SURVEILLANCE BROADCAST DISPLAY WE NOTICED AN ACFT APCHING FROM THE S AND SOON HEARD THE APCH CTLR CLR AN ACR JET FOR A STRAIGHT-IN APCH TO RWY 35R. THE CTLR THEN ASKED US IF WE HAD THE TFC IN SIGHT. AFTER LOOKING AT THE AUTO DEPENDENT SURVEILLANCE BROADCAST DISPLAY AND OUTSIDE; WE CALLED THE ACFT IN SIGHT. IT WAS WELL TO OUR S AND IT LOOKED LIKE WE WOULD BE WELL IN FRONT OF THIS ACFT. THE CTLR THEN TOLD US WE SHOULD BE PASSING BEHIND THIS ACFT AND TO MAINTAIN VISUAL SEPARATION. HE THEN SAID WE COULD WIDEN OUT TO THE L IF AS DESIRED. IT LOOKED TO ME THAT THERE WAS NO WAY WE COULD PASS BEHIND THE ACFT UNLESS WE DID A 360 DEG TURN OR TURNED S TO MEET IT HEAD ON. ANOTHER OPTION WOULD BE TO MAINTAIN ALT; FLY W AND DO A 270 DEG TURN BACK TO THE RWY CTRLINE. THE CTLR THEN ASKED IF WE HAD THE AIRFIELD IN SIGHT AND WE RESPONDED AFFIRMATIVELY. HE THEN CLRED US FOR A VISUAL APCH TO RWY 35L TO MAINTAIN VISUAL SEPARATION FROM THE TFC AND CLRED US TO LAND. BECAUSE WE WERE APPARENTLY WELL OUT IN FRONT OF THE TFC; THE CAPT ELECTED TO DSND AND CONFIGURE FOR LNDG. AS WE CROSSED THE EXTENDED CTRLINE OF RWY 35R; THE COMFORTABLE SEPARATION WE THOUGHT WE HAD ON THE TFC TURNED UNCOMFORTABLE. I DO NOT KNOW HOW CLOSE HE WAS. THE LAST DISTANCE I REMEMBER OBSERVING ON THE AUTO DEPENDENT SURVEILLANCE BROADCAST DISPLAY WAS APPROX 3 MI. BUT; WHEN OUR PATHS CROSSED; IT COULD HAVE BEEN CLOSER -- IT WAS UNCOMFORTABLE. LESSONS I LEARNED: WHEN CLRED FOR A VISUAL APCH AT NIGHT AND REQUIRED TO MAINTAIN VISUAL SEPARATION; I WILL BE EXTREMELY WARY. THIS SEEMINGLY SIMPLE MANEUVER IS FRAUGHT WITH CHALLENGES. THE MAJOR CHALLENGE FOR US THIS NIGHT WAS; IN MY VIEW; FATIGUE. WE WERE LEGALLY RESTED BUT OUR PREVIOUS LEG WAS FLOWN OUT OF OUR ESTABLISHED SLEEP REST CYCLE. REST FOR THIS LEG WAS MARGINAL AT BEST. I FELT FINE STARTING THE LEG BUT ABOUT THE TIME WE STARTED DSCNT; MY BODY WAS SAYING 'YOU CAN SLEEP NOW.' AS A RESULT; I WAS NOT UP TO PAR MENTALLY AND DID NOT ASSERTIVELY SUGGEST ALTERNATIVE FLT PATHS TO THE CAPT. THE SECOND CHALLENGE IS THE DIFFICULTY OF ASSESSING CLOSURE RATES AND RELATIVE POS AT NIGHT; CHALLENGING NORMALLY BUT ALMOST IMPOSSIBLE WHEN YOU ARE TIRED. THE REMAINING CHALLENGE I WILL DEFINITELY CONSIDER IN THE FUTURE IS THE ONE OF NOT KNOWING WHAT IS GOING ON IN THE COCKPIT OF THE CONFLICTING ACFT. THINKING BACK; I SUSPECT THE OTHER ACFT PLTS WERE TIRED TOO AND HAD DECIDED TO MAKE BEST POSSIBLE SPD ON THEIR FINAL APCH. WHAT LOOKED GOOD AT A DISTANCE WAS RENDERED POTENTIALLY UNSAFE BECAUSE THEY WERE COMING IN FASTER THAN I ANTICIPATED. THE SOLUTION FOR ME IN THE FUTURE; GIVEN SIMILAR CIRCUMSTANCES; WILL BE TO REQUEST A VECTORED FINAL APCH.