The stock boost gauge is rated in torr (or millimeters of mercury (100xmmHG)), 1 bar is roughly 700-760 mmHG, which is about 14 PSI. Halfway up the positive side of the gauge is about 7 PSI.
With the stock solenoid, it should read just under the bar between when the turbo spools up and 4500RPM, then just either dead on or just over the line the rest of the way.
I've found my gauge to be rather accurate once settled, but it isn't lightning fast to respond to spikes, I've been pretty good at telling when it spikes based on the speed of the movement to the next position.
My car is a pretty stock S2 R33 and I have been messing about with it for a while now. Unless your turbo is in a really shit condition or some idiot has installed a fuel cut defender I wouldn't worry about playing around with it too much.
You'll get a nasty jerk if it spikes into ~12 PSI as you'll hit a safety cut, just ease off the throttle and adjust the controller down. When testing your pressure, always check that you arn't hitting the safety cut in 4th or 5th gear as well. While I was playing about with it I had a setting where it would be perfect in 1st, 2nd and 3rd, but under more open throttles in 4th and 5th it would edge on cutting out.
If your gauge is accurate, you want about 10 PSI which is about halfway again between the +7 mark and half positive.