At the 300 lux setting the light will come on when the lighting level is roughly equivalent to that in, say, your kitchen when it's lit only by artificial lighting (i.e. when it's very easy to see).
At the 30 lux setting it should come on when you'd be struggling to read a book with that level of lighting but you can stil easily see what's going on around you.
At the 3 lux setting it should come on when it's almost totally dark.
Set it to '30' and, if necessary, adjust it later!