Okay: as of just after noon today, the water tank is absolutely as full as it can be*. One more molecule would fall right out the overflow pipe.
I just took this picture: The pressure is higher first thing in the morning for some reason, but 19.25ish psi in the middle of the afternoon is pretty high.
I’ll bet it reads 20 psi or better tomorrow morning, which will be an all-time high. So it’s official: Big Brother’s prediction was correct and pressure at the sink has a direct correlation with water tank level, and with a big enough scale you can use it as a warning of problems at the tank.
* For the record that’s 2500 gallons, about 50 feet above the Lair’s sink.