MLG FLC HAS ALTDEV REQUIRING VECTORS TO AVOID SMT.

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APPROX 130 DME S OF RIC ON J-165 CRUISING AT FL370. ZDC ASSIGNED DSND AND MAINTAIN FL330; CROSS 80 S OF RIC AT FL270. CAPT READ BACK CLRNC; I SET 330 IN ALT WINDOW AND STARTED DOWN. I PROGRAMMED FMS TO BE AT STOSH/-4 (WHICH IS RIC-80) AT FL240 AND THEN SET R40 IN THE ALT WINDOW. I WAS THINKING FL240; I GUESS; DUE TO THE XING RESTR BEING 4 SHORT OF STOSH. DSNDING THROUGH FL250; WE GOT A FREQ CHANGE AND I CALLED 'FL250 FOR FL240.' CAPT ASKED ME 'WHEN WERE WE CLRED TO FL240; ARE YOU SURE IT'S FL240?' HE HAD BEEN GETTING OUT HIS DCA APCH PLATES; I RESPONDED THAT I WAS SURE THE CLRNC HAD BEEN TO FL240. CAPT CHKED IN WITH ZDC 'LEVELING AT FL240.' CTR HESITATED; THEN RESPONDED 'ROGER.' APPROX 5-10 SECONDS LATER; CTR SAID 'TURN L IMMEDIATELY TO APPROX 300 DEGS W.' TCASII DID NOT SHOW ANY CONFLICT. AS WE ROLLED OUT ON A NW HDG CTR SAID WE HAD KING AIR TFC AT 3 O'CLOCK AND 10 MI. ABOUT THAT TIME THE TCASII DID SHOW HIM AT 3 O'CLOCK AND 10 PLUS MI. CTR SAID THE PREVIOUS CTLR HAD ASSIGNED US FL270 AND THAT HE HAD MISSED THE READBACK. HE THEN CLRED US TO SLOW TO FL250 AND PROCEED DIRECT RIC ON COURSE. REMAINDER OF FLT WAS UNEVENTFUL. I WAS SURE WE'D BEEN CLRED TO FL240; BUT OBVIOUSLY I HAD TRANSPOSED SOME NUMBERS. SUPPLEMENTAL INFO FROM ACN 317223: THE COPLT CONCLUDED THAT HE MUST HAVE GOTTEN HIS NUMBERS MIXED UP WHEN HE WAS FIGURING A LOCATION FOR THE '80 MI S OF RIC' TO PUT IN THE FMS. CLRLY THIS CASE EMPHASIZES THE NEED FOR THE 'OTHER GUY' TO 'MIND THE STORE' WHEN A PLT IS WORKING WITH THE FMS.