DISCLAIMER: UNPLUG THE MACHINE BEFORE MAKING ANY ADJUSTMENTS. ELECTROCUTION IS DETRIMENTAL TO YOUR HEALTH.
I tried boiler pressure settings from 0.7 to 1.3 bar and settled on the original factory setting, which was 0.9 bar in my case (measured from the top of the cycle). You can change the boiler pressure and thus the temperature by turning the 7mm nut atop the pressurestat spring (tighter = hotter):
The brew pressure is indeed completely different than the boiler pressure. Since Junior has no brew pressure gauge, you must use a portafilter pressure gauge, similar to the one below:
A brew pressure of 8.5 bar is a good starting point. Your model (S1) has a vibration pump and presumably uses an expansion valve to regulate the maximum pressure. Jim Schulman explained the
mechanics of vibe pump pressure regulation that apply to your machine. I'll defer to Chris' technicians for the specifics, since I had the DT1 (rotary) model and thus don't have any pictures.
Begin by lowering your pressurestat setting and then look to the brew pressure. Once you have the pressurestat setting correct, you can pretty much ignore the boiler pressure gauge. Junior's heat exchanger and boiler are large enough that the position of the needle at any given time is inconsequential for the purposes of pulling shots. The flushing regime I detailed in the guide should get you started, then you can fine-tune it to your tastes and blend selections.