AN ACR LGT CAPT FRETS ABOUT HIS SEPARATION ON TKOF BEHIND A B-757.

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ISSUED TKOF SEQUENCE BY ORD TWR BEHIND B-757. AS 757 TOOK POS ON THE RWY; A DC-9 BLOCKED OUR PATH TO THE RWY. TWR ASKED THEM TO PULL AHEAD TO ALLOW US ACCESS TO THE RWY AND THEN TURN AROUND TO FOLLOW US. AS THEY WERE MOVING OUT OF THE WAY; THE 757 WAS ISSUED A TKOF CLRNC AND AS HE BEGAN ROLLING; WE WERE CLRED INTO POS AND HOLD. I WAS DISTRACTED BY WATCHING THE DC-9 SWING AROUND CLOSE BEHIND US AS WE ROLLED ONTO THE RWY; WHILE LISTENING TO THE FINAL TKOF CHKLIST ITEMS BEING COMPLETED; AND MENTALLY PREPARING FOR A HVY WT TKOF VERSUS DEP CLRNC LIMITATIONS. WE WERE GIVEN A TKOF CLRNC AS THE 757 LIFTED OFF AND WHILE WE WERE STILL 90 DEGS TO THE CTRLINE; JUST STARTING TO LINE UP. I WANTED TO MAKE USE OF OUR FORWARD MOMENTUM AND BEGAN TO ADD PWR AS THE ACFT BECAME CLOSE TO RWY ALIGNMENT. AT THIS POINT I WAS CONCENTRATING ON SETTING MAX TKOF PWR AS SOON AS POSSIBLE WHILE BEING SMOOTH WITH THE NOSEWHEEL STEERING SO THAT BY THE TIME I LOOKED UP AND REALIZED THAT WE SHOULD HAVE 4 MI SEPARATION BEHIND A 757 WE WERE ALREADY THROUGH 80 KTS. THE 757 MADE A VERY SHORT TKOF ROLL AND THEN MADE AN IMMEDIATE L TURN. OUR CLRNC WAS TO MAINTAIN RWY HDG. RATHER THAN RISK A HIGH SPD ABORT ; I CHOSE TO CONTINUE THE TKOF BECAUSE I KNEW WE WOULD BE BELOW VR AS WE PASSED THROUGH HIS LIFTOFF POINT AND BECAUSE OF HIS L TURN; I FELT THE CHANCES OF ENCOUNTERING HIS WAKE WERE REMOTE. THE TKOF WAS NORMAL AND NO WAKE TURB ENCOUNTERED. AS WE CLBED OUT I POINTED OUT OUR MISTAKE TO THE FO AND SO AND THEY AGREED WE SHOULD PROBABLY HAVE QUESTIONED THE SEPARATION; BUT IT HADN'T OCCURRED TO THEM UNTIL I MENTIONED IT. ACTUALLY; I'M NOT SURE IF THE 4 MI SEPARATION BEHIND A 757 IS AN ACTUAL RULE OR JUST AN ATC GUIDELINE; AND IF A SHORT TKOF ROLL AND A STEEP; IMMEDIATE L TURN AFFECTS THE 4 MI RULE. IF THE ACFT WAS A HVY 747 NO QUESTION I WOULD NOT HAVE ACCEPTED THE CLRNC AND PROBABLY WOULD NOT HAVE GOTTEN THAT TKOF CLRNC; BUT OBVIOUSLY THE B-757 ISSUE IS NOT CLR TO ME OR THE CTLR WORKING TWR.