You also need a gauge on the line feeding the tank, so you'll know your cuton/cutoff pressures.This past winter I blew out the lines after draining the water lines, water heater, pressure tank. After shutting the well pump off thru the circuit breaker. Guage on pressure tank still read 40 PSI. A neighbor said that's normal because it's a bladder system. I thought that he might be wrong at the time...
Turn a faucet on, note the pressure your pump kicks on, you'll hear the pressure switch click. Then turn the faucet off and let the tank fill and note the cutoff pressure. Turn the pump off and drain the tank completely, noticing if you get air out toward the end. Then set the precharge pressure correctly, 2 - 5 psi less than the cuton pressure. Then turn the pump back on and let it fill the tank. Do not adjust the precharge except when the tank is empty.