Duh, why didn’t I think of that? Hang a picture/object level by putting painter’s tape on the level and marking where the object’s holes are. Put the level against the wall and mark the wall based on the painter’s tape markings.
Hang pictures without the trial and error: 1. Trace pictures onto brown kraft paper and cut out. Label each of the papers with a description of the picture or a corresponding number. 2. Using blue painter's tape (which won't pull up wall paint), tape the papers to the wall. Experiment with arrangements until you have one you like. 3. Install picture-hanging hardware directly through the paper. Pull paper away and hang pictures one by one.