As far as I know you cannot have boost lower than what your wastegate spring is set for. Unless you can figure out a way to make the wastegate open sooner.
I would probably just purchase or borrow the proper spring for 6 psi to be sure you'll have the proper boost level.
2012 Fusion SE 3.0L V6
'86 XR4Ti ; 2.3L Turbo, 15psi
1988 F150 Custom
'07 ST ; formally bewst'd - R.I.P
D23 Crew: #07