Put a level on the back deck side-to-side and then match your transducer to that. Next time your out put a level on your gunnel or somewhere front-to-back and when you get home put the boat back at that level. When it's back at the same level it was while you were idling go back and make your transducer level with the earth. That should give you the best picture
If your boat is sitting level on the trailer, I would put a small level on the end of the transducer to get it level with the ground provided you are on level ground. It looks to be pointing still too much to the starboard side from that view.