Some common ways of hanging paintings is to place the center at 1,5m from the floor, or to follow an invisible horizontal line of all the upper or lower sides. I have a lot of paintings in different sizes at home. Also I like to change the hanging all the time which makes these kinds of hanging methods a little complicated (if I dont want to make new holes in the wall all the time). One solution is to buy or make a painting shelf! Then you are automatically going to get a perfect lower horizontal line.
Isn't the wooden shelf above magical! It doesn't have any outher ledge though. Then you have to make sure the paintings are stable enough not to fall over the edge.
You can mix paintings with books and smaller sculptures to make it more dynamic.
I am going to look for a white really long ledge for myself.
Picture 1 from here, 2 from here, 3 from here and make your own 4 using this description.