I despise these types of brackets for hanging frames on the wall.
Place your frame against the wall in the place you wish to hang it. You can use a level if you like, but I usually just eyeball it, especially since none of the walls or ceilings in my house seem to have square corners or straight lines. When the frame is in the place you want it, press on the frame so the tape transfers to the wall.
Remove the frame and what you should have left is tape on the walls, exactly in the places where you need to drill the holes or put in the screws.