i would get an egt of fuel pressure gauge over an a/f gauge...

an egt will truely tell you how lean or rich you are

and a fuel pressure gauge will tell you if your fuel pump is spiking, or not flowing enough gas

and while you are at it.. if you are still on the stock pump id get an aftermarket one.. since fuel pressure almost doubles on nitrous (around 40 off nitrous and close to 80 on nitrous)