how to calculate apparent size of an object based on distance

I would like to know what the size of my monitor would appear to be if I were to move further away from it. It is a 22 inch monitor, and let's say right now I am 24 inches away from it. If I were 30 inches away from it, what would the size appear to be? I have an eye tracking device mounted on my monitor and it needs to know the size of my screen. If I move the device closer to me but keep the monitor in the same place, I will need to give it different measurements, because it will be as if I have a smaller screen.