Multiplying the current bill by a growth rate captures steady demand but misses scheduled change. End-of-support dates, log growth, recovery tests and migration overlap are predictable. If they are absent from the plan, teams meet them later with less time and fewer options.
Separate run cost from change cost
Baseline compute, storage and transfer should not hide projects such as a runtime upgrade, region launch or data migration. Different owners and decision dates apply.
Put support deadlines on the calendar
Managed service versions, operating systems, certificates and vendor contracts create hard dates. Budget engineering time, test environments and temporary capacity before the deadline.
Budget for verification
Backup storage does not prove recoverability. Include isolated restore environments and staff time for exercises. Observability also grows with retention, cardinality and duplicated telemetry, not only traffic.
State assumptions next to the number
Every major line should carry its usage assumption, owner, decision date and sensitivity. This lets the team revise the model when product or architecture changes instead of rebuilding it from scratch.