OK Matt,
Here's the Dew Point Table I promised. I've also included a graph of the same data - the waviness in the lines of the graph is just rounding error from the data. In reality they should be straight, but we're only talking 1 degree resolution here!
Striker I also appreciate your very practical advice (that's why I use two power tanks) but I'm happy to explore the science of the whole situation. You never know, someone might learn a little bit that helps them to make a wise decision somewhere down the track. It's all just little pieces of the big jigsaw!
Al.