The pressure should be set to ~12-15psi when cold. It seems this system has been over-filled (by error, or by a seeping auto-fill valve, or possibly a leak in the isolating heat exchanger shown in in the picture in response #3.)
An idling pressure of 20-25 psi isn't usually necessary unless the building is 4+ stories tall, and it will require re-fitting the pressure relief valve on the boiler to something higher than 30 psi. Most boilers are rated for 50psi (max) operating pressure, but are normally shipped with 30psi pressure relief valves.
With the system down to a safe (<120F ) temperature you can bleed of system pressure from any operable purging port until the gauge reads 12 psi. There is usually a valve with a hose connection somewhere on the near-boiler plumbing. If there are no purging port valves, you can alternatively manually operate the pressure relief valve (with the flip lever on top ) to bleed system pressure. Try that first, then fire it up, and note where the pressure peaks at the end of a burn cycle. It may be just fine, staying below 25 psi, and WELL below the 30 psi of the pressure relief valve.
The air preset on the tank needs to be whatever you set the system pressure to. Setting the pre-charge on the tank can only be done when the system side of the tank is de-pressurized. If the tank can be isolated with valves, the pressure on the system side of the tank can be bled off by unscrewing it partially to letting a some water dribble. The amount of water that comes out could be considerable (on the order of a quart or more) if the system is at 20 psi and the tank was pre-charged to 12 psi. Check the air pressure only after that significant water has been bled, and adjust as-needed with an air pump.
The tank should only ring when tapping on the end with the air valve. The end plumbed to the system will be full of water, and will deliver more of a thud than a ring.