Correct pressure for expansion tank?

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Providence, Rhode Island
I am replacing my thermal expansion tank on my Navien combo-boiler system. I've heard two different things about the 'correct' pressure for the expansion tank:
1. The main water line pressure (from an external hose spigot this is 45psi)
2. The pressure within the closed heat system (much lower...I've heard this is ~13psi).

Right now I have my new tank charged to 45psi, and I'm finding that the pressure relief is getting activated, as the boiler is reaching 30-35psi while active. This is leading me to think that #2 is correct. But, the alternative is that I did not set the pressure correctly inside the boiler when I purged the air out.
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