I have been lucky enough to have a few rain storms come through my location. I own an official NWS rain gauge and it is very near where I have the WS-2902A sensors installed. This allows me to tell how well the WS-2902A is measuring rain.
By default, it over-reports with the default calibration setting of 100. I sat it down to 90 and waited for the next couple of rain storms. This got it real close, but it was still slightly over. Right now, I have it at 87 which seems to be the magic number at the moment. Soon it will be snow so I will not know until next year when the rain returns.
On the calibration of the other sensors, I think these are actually perfectly fine. I see no need to change the temperature or humidity calibrations. The solar sensors are fine as well at their defaults.
You don't need to calibrate the barometer. You just set it like you do with the clock via the SET button. Once you have your MSL barometric pressure figured out, the WS-2902A really works well and agrees with nearby airports. This has become my favorite weather station I have ever owned.