I didn't say that it would be the same as the "normal system pressure". I said the "then-current system pressure".
You seemed to have been wondering what the internal pressure of the tank might be after the WH recovers. You said "If the water heating up causes the tank to become half filled with water, what is the pressure at that point? Is it 120psi because the air now occupies half the volume it did before?"
What I am saying is that the pressure in the tank will always be the same as the *current* pressure throughout the system. You can test that at any fixture within the system. I test at my utility sink since it has faucet with hose threads, making it easy to put a pressure tester on it.
That's the whole point of an expansion tank and the importance of proper pre-charging. The expanding water causes the baffle to compress the air, thereby essentially "enlarging" the system. The same amount of water in a larger system means lower pressure throughout.
Can I tell you how/if it will work with your system? No. All I can tell you that the addition of the tank in my house reduced the increase in pressure from 90+ after recovery to about 75. That's good enough for me.
It may or it may not. You can only determine that by testing the pressure. There are other ways to eliminate excess pressure, such as air chambers - vertical stubs of standard copper pipe that are filled with air. Unfortunately, they do not have baffles and can get filled with water over time, eliminating their effectiveness. Draining and refilling the entire system can often solve the waterlogging issue.
BTW...just as an aside, the Watts gauge that is shown in the expansion tank video that I linked to can be somewhat deceiving. You'll notice the red needle that is supposed to retain the highest pressure that the system reached. That needle can show a ridiculously high false reading if you have any water hammer. Here's the issue:
The red needle is "freewheeling" meaning that it is pretty loose on the shaft. That allows it to be pushed up by the black needle. There is just enough resistance to hold it in position after the black needle "recedes". However, there is not enough resistance to stop it from being *thrown* upwards when water hammer causes the black needle to swing violently. IOW, the black needle may jump from 60 to 80 when water hammer occurs. When the black needle hits the red needle, the red needle keeps going past 80 like a ball off of a bat. I watched my black needle jump to 80 psi and the red needle shot to over 110. That can be very deceiving if you aren't watching (like I wasn't) when the water hammer occurs. You walk up to the gauge and see a max pressure of 110+ and think that system is as spiking to 110+ which can be quite concerning.