The air/fuel meters you can get for like 40 bucks do nothing more than display a graphical representation of the voltage coming off the oxygen sensor. In and of itself, this isn't an issue. The problem is that the oxygen sensor is only really accurate under conditions which cause the ECU to go into open loop mode (high TPS and RPM values). When you're just putzing around the oxygen sensor reading will bounce all over the place and whatever shows up on the a/f meter doesn't mean a damn thing. Basically it's good only to show if you're leaning out on a WOT run, not as a tuning tool.

As for "tuning" a car with just an adjustable fuel pressure regulator...it's pointless. All you're doing is richening or leaning out the entire fuel map stored in the ECU, and only in very small amounts. Injectors flow a fixed amount, the way you get more fuel into the motor is to alter the signal going to the injectors and tell them to stay open for longer. This is how the ECU controls things, and this is why the stock fuel pressure is fixed at around 43 psi. To really see any gains you'll need some way to adjust the signals being sent to the fuel injectors (an A'PEXi S-AFC works well), a dyno and a wide-band oxygen sensor which most dynos are equipped with.