When you create a window you specify its size and position. Alternatively there's GetWindowRect in the WinAPI.

If you've already got x1,y1 and x2,y2, suppose x1=y1=100 and x2=y2=200. What would the centre point be? How would you work that out as an equation using just x1,y1,x2,y2?

It's impossible to forget x1~y2. There are no WinAPI functions that return the centre point of windows (or screens - do not confuse these terms otherwise your question becomes ambiguous. There is a definite difference between a window and a screen). The reason is that you can easily calculate that from the screen size and position, i.e. x1~y2.

u can get a software for measuring the screen size exactly. u can try that. It will give u the size in pixels.