B767 CAPT DESCRIBES QUESTIONABLE VECTORING AND SPACING ON A VISUAL APCH TO SEA.

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ARRIVED IN THE SEATTLE AREA AT AROUND XA15L. AFTER PASSING OLM VOR ON THE OLM 5 ARR WE WERE ASSIGNED A 030 DEG HDG AND A DSCNT TO 7000 FT. AS WE CONTINUED NE TOWARD TIFYS INTXN; APCH CTL ASKED IF WE HAD TFC IN SIGHT AT 2 O'CLOCK POS AND; HAVING THE TFC IN SIGHT; I RESPONDED WE DID AND WE WERE TOLD TO FOLLOW THAT TFC. THE TFC WE WERE TO FOLLOW WAS FOR RWY 34R AND WE WERE TO FOLLOW HIM TO RWY 34L. THE FO; WHO WAS FLYING; TURNED R TO ABOUT 040 DEGS OR 050 DEGS FOR SPACING AND SLOWED TO 160 KTS TO FOLLOW THE TFC WHICH STILL APPEARED QUITE A WAYS OFF. COMPLETING THE CHKLIST ITEMS I NOTICED WE WERE ALREADY ON THE LOC FOR RWY 34L AND; NOT WANTING TO CONFLICT WITH THE TFC FOR RWY 34R; WE INTERCEPTED THE LOC FOR RWY 34L. AT THAT POINT APCH CTL GAVE BOTH ACFT INSTRUCTIONS TO SLOW TO 170 KTS AND HANDED US OFF TO SEA TWR. ON CONTACTING THE TWR WE WERE CLRED TO LAND ON RWY 34L AND ADVISED THE TFC FOLLOWING WAS 1 MI BEHIND. WHEN THE FOLLOWING TFC CAME ON FREQ TWR ASKED IF THEY WERE OK LNDG BEHIND US AND THEY SAID THEY 'THOUGHT IT WOULD BE OK.' THE LNDG WAS NORMAL AND THERE WAS NO TFC CONFLICT AS A RESULT OF THE INCIDENT. THERE WERE A FEW FACTORS THAT LED UP TO THE INCIDENT. I FEEL THE ANGLE AT WHICH THE TFC WAS CONVERGING MAKES IT HARD TO JUDGE CLOSURE RATES AND RELATIONSHIP TO OTHER ACFT. WE WERE INITIALLY HDG 030 DEGS PRIOR TO OUR TURN TO THE R AND THE OTHER TFC WAS HDG APPROX 300 DEGS. ALSO THE TIME OF DAY (DUSK) MADE THE EXERCISE HARDER. ANOTHER FACTOR WAS THE ROOM WE HAD TO MANEUVER PRIOR TO LOC INTERCEPT. WE SHOULD HAVE TURNED TO A RECIPROCAL HDG (161 DEGS) TO THE RWY RIGHT AWAY IN ORDER TO POS OURSELVES BEHIND THE TFC. UNFORTUNATELY; WE WERE NOT ABLE TO JUDGE OUR DISTANCE TO THE TFC AND HIS CLOSURE RATE IN TIME TO MAKE THAT DECISION. IN MOST CASES; ON A VISUAL APCH YOU ARE SET UP IN A BETTER POS TO FOLLOW TFC; IE; ON DOWNWIND AND FOLLOWING TFC ON FINAL; TO BETTER JUDGE WHEN TO TURN IN OR TO SLOW TO FOLLOW. IN THIS CASE I DO NOT FEEL WE HAD THAT POSITIONING. SINCE WE WERE ON A VISUAL APCH TO FOLLOW TFC; ATC DOES NOT NEED TO ISSUE VECTORING OR SPACING GUIDANCE. HOWEVER; IT WOULD HAVE HELPED IF THE CTLR HAD LET US KNOW IT WASN'T GOING TO WORK OUT. MAYBE HE EXPECTED US TO FLY THROUGH THE LOC. HOWEVER; THE ACFT WE WERE TO FOLLOW WAS COMING FROM THAT DIRECTION AND I FELT A TURN SO CLOSE TO BOTH LOCS WOULD CREATE A CONFLICT. WE ACCEPTED THE CLRNC TO FOLLOW THE TFC AND; DUE TO THE FACTORS I MENTIONED ABOVE; WE WERE NOT ABLE TO DO THAT. THE PLT IS RESPONSIBLE TO MAINTAIN SEPARATION ON A VISUAL APCH. HOWEVER; I LET US GET PUT IN A SITUATION WHERE WE WERE NOT POSITIONED TO EVEN START THE VISUAL APCH. MY COMMENT UPON BEING CLRED TO FOLLOW THE TFC WAS 'HOW IS THIS GOING TO WORK?' I SHOULD HAVE GONE WITH THAT FEELING AND ASKED FOR VECTORS. I WILL BE MUCH MORE CAREFUL ACCEPTING VISUAL APCHS IN THE FUTURE.