Hello: Basically everything in the plumbing system, downstream of a pressure reducer is at the same pressure most of the time. That’s why it doesn’t matter where on the cold system an expansion tank goes. I prefer an expansion tank to a Governor 80, but if you have no place to put the expansion tank, than choices are limited.
A pressure gauge should be in every plumber’s tool box, but sadly it’s not, and that leads to guesswork 😕
Yours, Larry