FLT CREW OF B737 TURNED AT 6 DME VICE 8 DME WHILE FLYING RWY 34 MOUNTAIN DEP FROM SEA. PROC WAS MANUALLY ENTERED INTO FMC ACCOUNT NOT IN ACR'S DATABASE.

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CLRED SEA TO ZZZ ON MOUNTAIN 5 DEP; WHICH FLIES SEA 341 DEG RADIAL TO 8 DME R TURN 070 DEG HDG. THIS DEP IS NOT IN THE FMC. I MANUALLY ENTERED THE SEA 341 DEG RADIAL/8 MI FIX AS 341 DEG RADIAL AT 6 DME. MISTAKE #1. BECAUSE OF THE SHARP TURN; THE AUTOPLT; WITH LNAV ENGAGED; STARTED THE TURN A TOUCH TOO EARLY. WHEN WE TURNED EARLY DEP BROUGHT IT TO OUR ATTN (WE WERE STILL IN THE TURN AND TOLD US TO ROLL OUT AS HDG 050 DEGS AND STOP CLB AT 8 DME. NO UNUSUAL MANEUVERS WERE REQUIRED. I LEARNED A FEW THINGS: DON'T PROGRAM YOUR OWN DEP. WATCH OUT FOR AUTOMATION DEPENDENCE. WE NEED THIS DEP IN THE DATABASE; IF FMC/LNAV CAN FLY A HDG ON AN ARR AND HAVE FLY OVER POINTS; WHY NOT ON A DEP? SUPPLEMENTAL INFO FROM ACN 587700: WE DEPARTED BEHIND COMPANY WHO WAS CLBING MUCH FASTER THAN WE WERE. DEP CTL ADVISED US THAT COMPANY AHEAD WOULD BE TURNING R; AND CLBING OUT OVER US (WE WERE DEPARTING ON RWY 34R). WHILE WATCHING COMPANY AHEAD; I ENGAGED THE AUTOPLT AND LNAV; USING OUR MANUALLY INPUTTED WAYPOINT. I MISSED THE WAYPOINT ENTRY AT 6.0 INSTEAD OF 8.0 ON PREFLT AND WAS CONCENTRATING ON REMAINING CLR OF COMPANY TURNING INTO US DURING HIS CLB. I SHOULD HAVE CAUGHT THE EARLY TURN; BUT WAS DISTR BY DEP'S CALL ABOUT COMPANY TURNING TOWARDS US DURING CLBOUT.