HDG TRACK DEV DURING SID DEP PROC.

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COPLT FLYING FROM L SEAT; CAPT (ME) R SEAT AS IS COMPANY POLICY. I HANDED SID TO PF FOR REVIEW THEN WE BRIEFED THE DEP. WE WERE ASSIGNED RWY 34L FOR TKOF WITH COMMUTER ON RWY 34R. TWR RELEASED COMMUTER THEN RELEASED US FOR TKOF; TO KEEP THE COMMUTER IN SIGHT. NO PROB AS WE HAD HIM VISUALLY AND ON TCASII. THE PF WAS FLYING FMS TO THE 1ST 'FIX' WHICH WAS 8 MI; 4000 FT OR MORE. ALT WAS NO PROB; WE WERE ABOVE 4000 FT BUT PF STARTED TURN TOO SOON. BOTH OF US WATCHING COMMUTER; FMS COMMANDED TURN TO HDG 070 DEGS AS PER THE SID. I CHKED DME AND SAW 6.8 -- NOT 8.0 AS CALLED OUT ON SID. WE TURNED TO HDG EARLY. DEP ASKED IF WE HAD BEEN ASSIGNED SID. I REPLIED YES. HE REMINDED US WE NEEDED 4000 FT AND 8 MI BEFORE THE TURN E. WE WERE BOTH AWARE OF WHAT WAS TO BE FLOWN AND I CAUGHT THE EARLY TURN; REMINDED PF BUT WE WERE THEN ON THE HDG (070 DEGS). SEVERAL CONTRIBUTING FACTORS: FMS 'TURNED US EARLY;' BOTH PLTS WATCHING TFC ON R. NEXT TIME CALL TURN FROM R SEAT OR USE VOR/DME ON L SIDE NAV RATHER THAN FMS; TELL DEP OF ERROR (AFTER THE TURN; THE DME WAS 7.6 MI SO I THOUGHT IT UNNECESSARY TO TELL DEP); CHK AND DOUBLE-CHK ALL THE TIME.