There is a strong link between software quality and software reliability. By decreasing the probability of imperfection in the software, we can augment its reliability guarantees. At one extreme, software with one unknown bug is not reliable. At the other extreme, perfect software is fully reliable. Formal verification with SPARK has been used for years to get as close as possible to zero-defect software. We present the well-established processes surrounding the use of SPARK at Altran UK, as well as the deployment experiments performed at Thales to fine-tune the gradual insertion of formal verification techniques in existing processes. Experience of both long-term and new users helped us define adoption and usage guidelines for SPARK based on five levels of increasing assurance that map well with industrial needs in practice.