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Hi!
The way the IR was made (the driver and the hardware itself), it is enough to have one source of light, but having two (7 inches apart as in the original sensor bar) will make it a little better as it will know if you have rotated the control while pointing.
To make things easier, let's say you have just one led/candle/etc.
When not calibrated (or when calibrated to use all the possible area, up to the edges in all directions in the calibration window), the mouse cursor will be at the center of the screen when you point the wii control exactly in the direction of the source of the light, making it on the center of the IR camera.
Of course this is not good, because you want (with some accuracy) to make the pointer go exactly where you are pointing. That means that you should put the IR Leds in right the center of the TV screen, in front of the tube.
But you have to choose to put them above the TV or below the TV. This means that the mouse cursor will center on the screen when you point the remote a little above or a little below the center, and you have to consider this "offset" all the time.
This is annoying, but at least you have all the possible range of the control to use for more accuracy. If you want to make the cursor go to the center when you really point at the center, and still have the sensor bar above the TV, you will have to lose some area/resolution to compensate for this. The usefull range will be a little lower in the vertical axis.
That should be no problem for most users, but in my TV (a big LCD one) I found that the smaller range did make difference. With the sensor bar at the top, while calibrating, the light source hit the botton of the calibration window way before I really pointed the botton of the TV.
After some time I noticed that the farer from the TV I was, the less that mattered. The range get better when you go away form the sensor bar. The problem was that I was testing too close. After calibrating from sitting on the sofa, where I would really be using the remote, I was enough to be ok.
I hope that helps.
Andre