This article from Domino is an especially helpful resource for hanging groups of artwork. This one is my favorite.
Using a set height for pictures has really helped to unify all of the artwork in our house and keep them in the same line of vision. I use these guidelines from Apartment Therapy.
1) Measure 57 inches up from the floor. That is where you want the center of your picture to hit.
2) Measure the height of the frame, cut in half
3) Measure distance between hanger and edge of the frame, subtract that from half measurement of frame
4) Add that number to 57 and you will know where to put your nail
In hanging the hallway art gallery, I measured based on the height of all of the pictures together to ascertain my center point.