view on screen at actual size

I'm trying to view a very large photoshop file on screen at actual size. My 27" iMac with Retina display has a screen resolution of 5120 x 2880. Actual viewable screen width is 23.375", which ought too give me a screen resolution of about 219 psi. I plugged that into photoshop preferences/Unites & Rulers: Screen resolution. However, my ruler increments onscreen in photoshop still measure well over at 100% view. What am I doing wrong? Even when I divided 5120 by 27", it's still not calibrated correctly.