It doesn't make any difference whether the Tee is pass-thru or plugged on one side, as long as it is open to the pressure of the water line. Water rises and falls in the accumulator - it does not actually pass through it.
20 psi is a typical accumulator tank pressure. Since your water system operates at around 40-45 psi (governed by the max output pressure of the pump), an initial pressure of 20 psi allows the accumulator to partially fill with water until the pressure reaches around 40 psi. After that the level "floats" as pressure surges through the water line. If the pressure was very much higher than 20-25 psi, the accumulator may not function as designed. If much less than that, it will become "water-logged", i.e. have too much water and not enough air.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL