screenwidth and screenheight measure the width and height of you screen not your level. that means that if your resolution is 800x600, screenheight returns 600 and screenwidth returns 800. likewise many computers use 1024x800, in which it would return a different value. screenwidth and screenheight return different values based on computer resolution.
the way you want to do it is based on tiles. the setfocus command doesn't work off of the position relative to the screen, it works off the position relative to the level. if you try setfocus 32,32; it should be centered in the screen. the reason your equation worked is because your screen size just happens to be the right size... for others it may not work so nicely
