Dean / Shelleycat / Techone,
All thanks for you responses. Including the voltage drop in the calculation makes sense. Made my first circuit and I understand it! Now continuing with a more difficult circuit: both series and parallel...
One more thing about calculating the voltage of the voltage divider... Dean said to connect the multimeter at the junction and the ground, but I don't think I get the proper readings. I'm using two equal resistors and I expect to get a voltage reading of 3V, but the multimeter is reading 0v!? Maybe because there is no load connected? So I've put a led in the circuit but then the multimeter is reading a 1.80v? This is the same as the voltage drop of the led, so I don't think the multimeter is reading the correct value, or more likely its me using the meter wrongly!
@Techone: I'm a programmer, so programming the Arduino will not be a problem...