Percentage-based capacity models feel precise and fail in practice. Interrupt-driven work does not respect the twenty percent you reserved for “unplanned.” Support rotations, hiring loops, and production noise expand until the percentage is fiction.
Instead, protect named blocks on the calendar — discovery, build, and hardening windows — and measure how often those blocks survive intact. Survival rate is a better leading indicator than story-point velocity.
When survival rate drops, the fix is rarely “plan harder.” It is usually to renegotiate interrupt policy, shrink WIP, or staff the on-call load that planning ignored.
Capacity conversations improve when leaders talk about protected time lost, not abstract utilisation.