Hey Tom,
The big dial is basically useless as far as I understand. It will stay at 550 until it runs out of liquid CO2, and then it will start to drop quickly. The pressure on the big dial will really depend on the temp too... room temp will make it higher. Mine is sitting in the fridge at 8 degrees and reads 800.
I'm not so sure on question 2... is it hooked up to a keg, and in the fridge?