Ah! Yes, that is a tough one. It's been a few years since I fidled with that. I borrowed the Merc manual to adjust my original cable. I remember that the cable makes the shift, and the cable housing makes the ignition interupt.
When I decided to replace it I took it to a friend that has a boat repair shop in Garfield, MN, so I didn't have to adjust the new one.
I hope the adjustment goes well for you.
PeterJ