I'm assuming I'm right in thinking with auto brightness enabled the iPad controls the brightness with it's light sensor. If that's the case then why when it is enabled and I move the slider this overrides the auto battery. So what are the best settings slider down and auto or slide up and auto what's best for battery cheers

Having auto set on does allow the iPad to dynamically adjust the brightness, but it only does this within a limited range either side of where you have the brightness slider set to. Therefore if you have the slider all the way up or down you won't get any real benefit from auto.

For me, I find the best balance of battery life versus screen brightness is to have it between 50% and 60% (depending what I'm doing) with auto enabled. I find I really notice the battery impact if I have brightness set above 70%, so that's only for special occasions!

YMMV.

My understanding of how this works is this... You set the brightness to the desired level to your liking and then auto brightness will attempt to mimic that effect based on the current lighting situation. Basically, making the screen always look like it has the same brightness to you no matter how the room is, if that makes any sense.

In other words, you adjust the brightness to your liking and it will attempt to maintain that same brightness relative to whatever the currents room lighting is.

What Justin says makes sense, so to answer your question, lowest brightness and auto would be best for you. Personally I have auto off and leave it at lowest to conserve battery life (because I'm constantly using it for school between 8am and 9pm) and increase the brightness only when i need it (for glares or videos).