All wastegates come with a set pressure spring. i.e. you can set it so that it's 5,6,7,8,....etc. psi - that will be your lowest setting. then with a boost controller you can increase (only - not decrease) the boost. If you had an 8psi spring and found you need a lower base boost level you could take the wastegate apart and swap a less stiff spring for a lower base boost.
Ohsigmachi - you cannot adjust below the base setting of a wastegate's spring - that would defy the laws of physics.
