Understanding an air/fuel ratio gauge

okay this may sound like s tupid question to some, but how do you read an air/fuel ratio gauge ? I've got the Autometer one (the on with 3 wires) and it's all hooked up right and soldered together and everything. The oly part im not understanding is how to read it. At idle it jups all over the place and when im driving it does it to. I've heard that it only counts what it says when your at WOT. When im a WOT it reads RICH. I've heard for nitous use i want to be in the STOICH area. Also, when im downshifting coming to a stop the gauge shuts off. Is it supposed to do that ? If you could please help me, id appreciate it. One more question: Which other gauge should i get for the nitrous ? I'm thinkng oil temp, but i dunno. Thanks guys.