I've been trying to no scope it, its purely luck..though its generally pretty much dead center of the screen. The only real way to do it is kind of aim around and when you see the target's name pop up and glow...FIRE :-P
Of course, this is purely for medium to short distance whenever i'm too cheap to buy a deagle (which is far better at that range, anyway).
Dot on the screen sounds lame, but i dont think it'd work..lcd you dont want anything touching the screen, on a regular screen, the glass is a little bit off from the screen...ie, if you sit differently, you're gonna get a different pov, so the dot wont be accurate, anyway.