In the Irish Museum of Modern Art, as a general rule, paintings are hung with a centre height of 142cm (more or less 56 inches).
In all galleries around Dublin this is used as a standard for hanging at eye level, although with smaller work (under 80cm) the centre height is often raised towards 150cm.
The methodology to hang is quite easy.
You measure the height of the work. Divide by 2. Add your centre height that you have decided for the room (lets say 142cm). Take away the top height down to your hanging system (D-rings, stretcher frame or string for framed work) and this total height from the floor is your line to sprit level a line to guide two screws into your wall (only one in the middle if it is for a string).
If you do this for both works, they will have the same centre height whichever wall you hang them on in your room, assuming your floor is level. If you keep the same centre height throughout your room, it will generally look better.
I hope this helps.
optimum viewing distance?
I like 5 cm for details....