The wastegate spring sets the minimum boost pressure you'll be able to run, so with a 0.9BAR spring you will not be able to run any less than 0.9BAR (although after some pressure drop through the IC you boost gauge will probably read a little less). A boost controller, whether it be mechanical or electronic, can only be used to increase the boost above spring pressure. However there is a limit to the amount of pressure increase a boost control can reliably produce, the general rule is to use a spring pressure that is rated to at least half the pressure you want to run using the boost controller. For example if you want to increase boost to 1.2BAR by using a boost controller then you should have at the very least a 0.6BAR spring in the wastegate, this gives you the opportunity to run 0.6BAR low boost and 1.2BAR high boost, however if you will never have the need to run 0.6BAR you would see a more reliable boost control from using say a 0.9, 1, or 1.1BAR spring. Basically you need to decide on the minimum boost pressure your going to want to run and then fit a wastegate spring that is rated at that pressure, but then also keep in mind that you'll be limited to a maximum of double the spring pressure when you decide to up boost by using a boost controller. Once you reach that maximum limit a higher spring rating is required to maintain reliable boost control.