If it helps... we have the
Jabsco 30573-0000 accumulator tank. The manual is available on the Xylem website. The part about charge pressure says:
TYPICAL INSTALLATION
PRECHARGE PRESSURE ADJUSTMENT
The 30573-0000 tank is precharged at 10 psi (0.7 bar).
If your pump cut-in pressure is significantly different, you
may custom tune the tank to best fit yourparticular
installation. To increase air pressure in tank, shut pump
off, open a faucet to relieve system pressure and adjust
precharge pressure using ordinary tire gauge and tire
pump at valve in end of tank. Pressure should be
checked from time to time. To check tank pressure: turn
off power to water pump, open a faucet to relieve
pressure. Check tank pressure, adjust if necessary.
Close faucet and turn on power to pump.
For best results, do not pressurize tank above 10 psi
(0.7 bar).
The manual doesn't clarify accumulator charge pressure relative to the water pump's cut in/cut out pressure... although it at least hints that the pressure setting should be set relative to cut in.
Our pump cuts at 20 and 40 psi. The pump manual is silent on the topic of accumulators. Experimenting with the accumulator pressure led me (eventually) to leave it at 10.
Air pressure in the accumulator can change if temps change dramatically (although sometimes that seems to be self-correcting as temps return to normal), or if the air bladder ruptures.
-Chris