HVT FLT CREW DIDN'T UNDERSTAND ALT AND DISTANCE ON CROSSING RESTRICTION. DURING REPEAT CTLR INADVERTENTLY GAVE WRONG ALT; BUT CAUGHT DEVIATION BEFORE LESS THAN STANDARD SEPARTION OCCURRED.

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AT XA20Z; 5/THU/88; FLT ATL-BWI; TALKING TO WASHINGTON CENTER ON 133.72. WE WERE GIVEN A CLRNC TO CROSS 85 MI S OF RICHMOND AT FL290. I DID NOT UNDERSTAND THE CORRECT MILEAGE FROM RICHMOND. I SAID; 'UNDERSTAND FL290 AT 65 OR 85 MI THIS SIDE OF RICHMOND.' ATC SAID; 'FL250 AT 85 MI S RICHMOND.' I SAID; 'FL250 85 MI FROM RICHMOND.' AT THIS TIME I CHANGED THE ALT WINDOW FROM FL290 TO FL250. AT AN ALT OF FL270 WE WERE ADVISED TO CLB BACK TO FL290. ALSO GIVEN A HDG OF ABOUT 340 DEGS UNTIL OUT OF FL280 AND THEN BACK ON COURSE. WE WERE AT FL280 AND CLBING SO CONTINUED ON COURSE. WE THEN WERE ADVISED OF AN ALT VIOLATION RPT THAT WOULD BE RPTED ON FLT AND WE COULD OBTAIN FURTHER INFO BY CALLING THE ARTCC. REMAINDER OF FLT NORMAL. ONCE ON THE GND; I CALLED THE WATCH SUPVR AT THE TIME OF THE INCIDENT. HE SAID THAT AFTER REVIEWING THE WASHINGTON CENTER TAPES; WE WERE CORRECT--THE CTLR HAD IN FACT ISSUED OUR FLT A CLRNC TO CROSS 85 DME SW OF RICHMOND AT FL250. SUPVR ALSO SAID THAT HE LET THE INVOLVED CTLR LISTEN TO THE PLAYBACK OF THE TAPES AND HE (THE CTLR) WAS QUITE SURPRISED AT THE CLRNC HE ISSUED. THERE WAS NO LTSS OF ACFT; SO THE INCIDENT WAS CONSIDERED TERMINATED.