I need one isolating my hot from cold, and another where the park's copper flex tube attaches to my mobile home.

"Street" pressure was in the low 60's when I read it earlier today; we'll see what the overnight peak is tomorrow.

There will already be a PRV (Watts LFN45BM1) in the system, bringing pressure down to 50, but that's only 10psi or so of headroom, and I don't think I should eat it all up with some huge honkin heavy spring double check valve down below.

Related, how do I tune the expansion tank; to the regulator pressure or do I subtract the nominal drop of the check valve?