You'll definately want to keep an eye on your fishfinder to see where the nooks are but for the coho I usually stay at the 20ft. mark. Nooks are "usually" deeper but last year for some reason we caught nooks up in the coho water all summer. Depending on where I'm at I will drop the riggers to 75-100ft. down when targeting nooks. This early in the season though they could be even 200+ ft. down. Just keep adjusting your depths till you find them. It's a little easier if you have more than one downrigger also.
Good luck! There's no better way to catch fish in the ocean than off downriggers!