The expansion tank needs to be adjusted to the cold system pressure when it is completely empty of water and isolated from the system. Only way to do it.

There is a major misunderstanding with the expansion tank pressure. You need to start at 12 PSI air pressure in the tank and set the boiler feed cold (ambient) temperature to 12 psi. They both need to match. [huge snip]

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The expansion tank needs to be adjusted to the cold system pressure when it is completely empty of water and isolated from the system. Only way to do it.

Thank you all for the helpful replies. It's been a hectic day. Let me answer a little and then try to take and post pics tomorrow.I bought a liquid filled gauge and attached it to the hose bib on the boiler. Surprisingly the original gauge is tracking right along with it.The expansion tank was originally 1/2 to 2/3 full of water. During my reconfiguration, I pumped it up to 20 psi. But then it only was filling the top 2 or 3 inches, so I let a little air out, bringing it down to 17 psi.When I let a little water out of the system, the pressure drops. But then it climbs right back even though I have adjusted the PRV. I need to try setting it to 12 psi tomorrow when it is at ambient.Again, thanks all, and I'll be back tomorrow.Jay

Step one. Check the gauge. The built in gauges sometimes don't tell the truth. You can get a gauge which goes onto one of the drain fittings at a big box....