I put the tattle on the outside faucet closest to the water heater. After three days, it shows 56 psi. I have another gauge that I moved around to the other two faucets and they read on average 42 psi, but when my wife runs the washer or turns on and off the tubs I observed the needle swing somewhat abruptly to 35-29 psi and then slowly climb back up to around 40; during the evening before bed if the water dispenser in the 'fridge runs I see the pressure go from 45 psi to 42 then pop up to around 50ish before settling back down to nominal.
The experts commenting on this thread indicate my pressure should not fluctuate much but I see a maximum of 27 psi change depending on what turns on/off; when a fast acting valve opens then closes I can hear a "ka-dink" in the crawl space that damps out slowly after 2-3 more ka-dinks. I've also read on another board someone who had the same symptoms (with average pressure) and changed the T&P but the leak appeared on the new T&P, so I'm thinking I need arrestors on (Terry's big three) culprits for hammer, a new expansion tank, the PRV needs to be replaced and my T&P is probably doing its job. I'd say that's just about right for this house.