When off plane, the drive should be all the way IN. Trim is really only used for being up on plane. There really isnt' a good middle ground, either your up on plane or your not. Halfway in between is just ridiculous and a huge waste of fuel. I'd start by counting the number of 'clicks' up and down the drive takes. Then once up on plane, start clicking the drive out once click at a time. Watch the Tach. and you should see the RPM's increase slightly. Keep doing this until either the RPMs stop increasing, or the drive cavitates. Click down immeidately if that starts, you're out too far. You'll get the feel for about how many clicks it takes. If you don't have tabs, you might need to trim in more when it rough to get the bow down.
Also note if your porpoising too much. That is usually a condition of too much trim out. trim tabgs or smart tabs would help this condition.