the spring in the actuator will resist pressure upto 7psi as t28 for example.

the T piece can hold 7psi and only give the actuator 3psi to allow the wastegate to stay closed longer increasing boost. to reduce boost you will therefore have to reduce the spring rate so the actuator opens on smaller pressure levels.

if the turbo boosts 7psi without T piece that is its default pressure it cant go lower than that. from what i understand and this is very difficult to try explain