From extensively testing this, the specific function that is being used to convert back from normalized coordinates to pixels is equivalent to:
x = trunc(dx*ScreenWidth/65536)
y = trunc(dy*ScreenHeight/65536)
So to make sure that the click arrives at the exact coordinates you desire, you have to convert from pixels to normalized like this:
dx = ceiling(x*65536/ScreenWidth)
dy = ceiling(y*65536/ScreenHeight)
In a multi-monitor setup, ScreenWidth and ScreenHeight are the width and height of the primary monitor.
Caveats: I have only tested this for positive values of x and y, and only on Windows XP SP3.