A B737 DEPARTING SEA DEVIATED FROM THE MOUNTAIN 5 SID BECAUSE THE FMC WAS IMPROPERLY PROGRAMMED.

Narrative

WE WERE CLRED THE MOUNTAIN 5 DEP; RADAR VECTORS TO J90; TAKING OFF TO THE N AT SEATTLE. AT THE GATE; I LOADED THE FLT PLAN WHICH HAD THE FIRST POINT OF NORMY ON THE JET RTE. THINKING I WAS HELPING THE DEP; I INSERTED THE POINT NEZUG FROM THE SID (SEA 341 DEG RADIAL 8 DME) AS OUR FIRST POINT AND THEN A 070 DEG HDG OFF OF NEZUG AS A SECOND POINT FOLLOWED BY NORMY. MY THINKING WAS THAT WE COULD BACK OURSELVES UP ON THE SID. I BRIEFED THE CAPT AS TO WHAT I HAD DONE. NORMAL DEP. AFTER WE WERE CLEANED UP; THE CAPT SELECTED THE AUTOPLT AND LNAV TO NAV THE SID. AT APPROX 7 DME N OF SEA; 4900 FT MSL; THE AUTOPLT BEGAN A R TURN TO A 070 DEG HDG; APPROX 1 MI EARLY. I VERBALIZED THAT WE WERE TURNING EARLY AND AS THE CAPT WAS REACHING TO DESELECT LNAV WE WERE CONTACTED BY DEP TO TURN BACK TO A 341 DEG HDG. WE HAD TURNED APPROX 30 DEGS BEFORE WE BEGAN THE TURN BACK TO THE 341 DEG ASSIGNED HDG. WHEN DEP WAS VECTORING US BACK TO THE 341 DEG HDG HE EXPLAINED THAT BEFORE WE TURNED WE NEEDED TO HAVE BOTH 8 DME/OFF OF THE SEA 341 DEG RADIAL AND 4000 FT MSL. THIS WAS PRIMARILY CAUSED BY MY PROGRAMMING THE 070 DEG HDG OFF OF NEZUG INTO THE BOX. HAD I JUST PUT NEZUG IN; JUST USED RAW DATA FOR THE POINT (SEA 341 DEG RADIAL; 8 DME); OR HAD WE NOT USED THE AUTOPLT; WE WOULD NOT HAVE TURNED EARLY I BELIEVE.