what you need to do is think about the final boost setting you wish to run at.
dont waste your time having a low boost setting and a high boost setting thats just silly.
pick your desired final boost level say 20 psi, and then get a new actuator as close as you can get to that level.
ie a 18 psi actuator.
once you have that make the shaft to suit the position thats its in so the rod is pulling central on the diaphram and set about half a hole prelaod on the gate pivot.
then you will have good, stable, boost control.
the main benifit i found is boost will ramp up real hard to 18 psi before the gate cracks open so the midrange is improved dramatically.
and by using just half to 3/4 a hole prelaod you will get max travel from the flapper and therfore have good wastgate flow up high in the revs.