I have about the same problem, I can't grasp feet at all and for screen sizes, centimetres would be easier with just an additional point for laptop screens, it's quite difficult to read indeed. I can represent a 15' screen thanks to my laptop but absolutely not a 40' or 50' screen, it would be better in centimetres with a dot for common screen sizes.
Just so you know, ' is pretty much always used to designate feet, " is inches. Source: I'm a mechanical engineer in the US.
What country are you from that uses CM for screen sizes? this space has pretty much been dominated by imperial units since pretty well forever...
I'm french. While people use inches for laptops, anything bigger is in centimetres.