Expansion tank replacement question

[rh71]

Have a 2-gal expansion tank on my indirect water heater that needs replacing. Problem is I'm getting a 90 psi water pressure reading at the pipe/valve that leads from it (same pressure the town sends water to us). What am I supposed to set the new expansion tank's PSI at since most "max air charge" at 80 psi? Will that difference matter that much and is it unsafe to do 80 psi in the first place? I do have a pressure reducing valve hooked up to the pipes/boiler though it's not in the direction of the water heater but the rest of the house.

This is what I bought and the sticker says max air charge 80 psi:
https://www.lowes.com/pd/EASTMAN-Th...el-Water-Heater-Pressure-Regulator/1000522213
Here's another for $20 more, not sure if I should've spent more:
https://www.homedepot.com/p/Amtrol-Therm-X-Trol-ST-5-Expansion-Tank-ST-5/202268963
Water heater with expansion tank placement:

Here's the pressure reducing valve that doesn't seem to come into play... still reads 90 psi at the cold water supply under the expansion tank (and washing machine, and outside spigot):

 

[breplum]

You might try your question at a heating help site, The Wall: Heating Help: The Wall
Code for domestic water in a house is max 75 psi. Pressure redicing valve is required to bring domestic water into range. There are many reasons for this.
No expansion tank will work above that range. Period.

As a plumber, I do know you need to address your high pressure. Hydronic systems are designed to operate at low pressures.

I also recommend that a tee with a hose bibb be installed between the shut-off valve and the expansion tank to allow for better replacement w/o mess.
Also, expansion tanks must be supported. That hanging tank is improper and at risk.
To set up and expansion tank (recalling from memory) you measure your existing pressure and then set the new tank pressure to match.
No way will 90psi work.

 

[fixitron]

That valve in the second picture is a feed pressure regulator designed to maintain a minimum (12-15 psi) pressure in a boiler system. Boilers have a 30 psi Pressure & Temperature relief valve.