FLC OF MLG ACR ACFT DEVIATED FROM ASSIGNED ALT IN RESPONSE TO A TCASII RA.

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WE WERE IN LEVEL FLT AT FL230; WAITING FOR A SBOUND (WHICH HAD BEEN CLRED TO DSND TO FL240) TO PASS SO WE COULD CONTINUE OUR CLB. WE GOT AN RA ON OUR TCASII 'DSND; DSND NOW.' THE SYMBOL WAS RED (PLUS 11) AND SHOWING A DSCNT. TCASII WAS ASKING FOR 2000 FPM OR GREATER DSCNT. WE STARTED DOWN OUT OF FL230 FOLLOWING TCASII COMMANDS. ZTL WAS ADVISED OF OUR RA AND THAT WE WERE DEVIATING. THE CTLR ANSWERED THAT WAS CLRED TO FL240. AT 22600 FT TCASII SAID 'MONITOR VERT SPD' AND ALLOWED NO CLB. THE PNF GOT THE ACFT VISUALLY AND ADVISED WE WOULD MISS. I WAS WATCHING TCASII AND NEVER SAW THE ACFT. PNF ESTIMATES WE MISSED BY 600 FT VERT; 500 FT HORIZ. WHEN TCASII SAID 'CLR OF CONFLICT' WE CLBED BACK TO FL230. WE ADVISED CTR WE WERE BACK AT FL230. HE ASKED IF WE HAD DEVIATED. WE SAID YES. HE MADE NO COMMENT AND HANDED US OFF. NEVER HEARD FROM XXX. WE HAD A NEAR MISS -- SO WHAT HAPPENED? WE FOLLOWED TCASII COMMANDS AND MISSED BY ESTIMATED 600 FT. HAD WE DONE NOTHING WE WOULD HAVE MISSED BY ESTIMATED 200 FT. ZTL HAD PROVIDED STANDARD SEPARATION AND WAS NO WAY AT FAULT. HOWEVER; IT MAY BE BETTER TECHNIQUE NOT TO CLB OR DSND HEAD-ON TFC TO ACHIEVE MINIMUM SEPARATION EXACTLY WHERE OTHER TFC WILL BE. OUR CLOSURE RATE WAS PROBABLY OVER 600 KTS. THE RA DIDN'T OCCUR UNTIL AT CLOSE RANGE. NOT MUCH TIME TO THINK ABOUT IT.