AWOL is this mantra always applicable when the inconsistency is this small or should I try to improve the design - given that I really don't have much idea about earth loops and the like. As this is my first experiment with anything electronic really, I'm all ears. As you say, this result is not bad and so I guess I should begin to focus on what the implicatons are. I'll take a look at what constitutes a shortfall in performance and what doesn't - given the right method of interpreting the result/s.
Just for your interest, there is really only one shot when it comes to reading the voltage from the sensor - this is because the colour is moving beneath it and any second read, should be labelled as 'second read' and compared with an 'ideal second read' value - at least this is my current thinking.
Thanks for the input, I'm getting a feel for the subject.