where to place outside movement detector for lights?

i want to get one of those 3 wire clipsal infrascan movement detectors, will i need 2??
see from the image below, i have a front yard that is 10m from house to roadway and 20 metres from side to other side. i have an electrician coming to do some other wiring and will get him to put one up but want to know where it will go first.
marked on diagram are 1, 2, 3. these are places where im thinking i can mount this movement detector(s). number 3 is my front door, dunno how good one will be there as theres a bush 1m high in the front yard that may obscure it a bit.
where would be the best place to mount one of these movement detectors?

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