ARTCC CTLR HAD LESS THAN STANDARD SEPARATION BETWEEN LTT DESCENDING AND WDB ENROUTE OPPOSITE DIRECTION TRAFFIC.

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WDB X EBND AT FL370. LTT Y WBOUND AT FL390. THE LTT Y REQUESTED LOWER SINCE HE WAS LNDG AT COLUMBUS; OH. THE CTLR KNEW IT WOULD BE CLOSE; BUT THOUGHT THE LTT Y COULD MAKE A GOOD RATE DOWN. AFTER 2 RADAR UPDATES; IT BECAME APPARENT THAT VERT SEP WOULD NOT EXIST AND AVOIDANCE VECTORS WERE ISSUED TO BOTH ACFT. IT WAS SIMPLY AN ERROR IN JUDGEMENT. DIFFERENT PLTS FLY THE SAME ACFT DIFFERENTLY; AND IN THE SITUATION I FELT IT SHOULD HAVE WORKED. WORKLOAD WAS HVY; BUT NOT UNMANAGEABLE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: RPTR SPECIFIED TYPE ACR ACFT AS WDB. COMPUTER ONLY HAD ONE HIT AT 4.3 NM AND SHOWED 1600'. CONSIDERED A MINOR INFRINGEMENT. TFC ADVISORY WAS GIVEN BOTH ACFT AND LTT HAD TFC INFO BEFORE DSCNT CLRNC WAS GIVEN. LTT WAS ASKED IF HE COULD BE LEVEL AT FL350 IN 1 1/2 MINS; AND HAD ADVISED AFFIRMATIVE. NTAP SHOWS LTT ONLY DSNDED AT 1500 FPM WHICH WAS NOT CONSIDERED OR EXPECTED TO BE A GOOD RATE OF DSCNT. SECTOR WAS STAFFED WITH A D CTLR AND SUPVR WAS STANDING BEHIND THE SECTOR AT TIME OF OCCURRENCE.