A C550 CREW DURING TAXI WERE GIVEN A NEW CLRNC REQUIRING FMS WORK AND SUBSEQUENTLY DEPARTED ON THE WRONG RWY.

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ON DEP FROM LUK; WE WERE TOLD TO BACK TAXI ON RWY 21L FOR RWY 25. WHILE TAXIING DOWN RWY 21L; I AS PNF; WAS GIVEN NEW DEP PROC. AS THIS PROC NOT IN DATABASE OR FMS; I HAD TO REPROGRAM; TOTALLY DISTRACTING ME DURING DEP ROUTINES. THIS ALSO LED TO PF GOING OVER CHANGED DEP PROC WITH ME; ALSO DISTRACTING HIM TO A POINT. WE COMPLETED TKOF CHKLIST AFTER RESOLVING THIS ISSUE. WHILE STILL TAXIING TO END; ON TWR FREQ WE WERE ASKED 'READY FOR TKOF?' WHICH WE WERE; AND I ACKNOWLEDGED AND READ BACK TKOF CLRNC OF 'CLRED FOR TKOF RWY 25; TURN L ON DEP TO 180 DEGS.' WE MADE THE 180 DEG TURN ON THE RWY AND TOOK OFF ON RWY 21L IN ERROR; NEITHER OF US REALIZING ERROR UNTIL CALLED BY TWR CTLR WHO ADVISED OF ERROR. NO CONFLICTS LUCKILY. THE CAUSE OF THIS AFTER SOME REFLECTION ON IT BY THE TWO OF US WAS: #1 -- THERE WAS DISTR OR CONFUSION ON MY PART DURING THE REPROGRAMMING OF FMS; LEADING TO RUSHING. THE BOTTOM LINE IS: NEVER; NEVER RUSH. PULL INTO HOLDING AREA; TAKE YOUR TIME AND DON'T GET OUT OF YOUR ROUTINE.