The short answer
Calculate them on a cohort, not on a snapshot. Take every opportunity created in a month, follow that same set forward, and measure what share reached each later stage. Snapshot ratios (stage two divided by stage one today) mix cohorts moving at different speeds and will mislead you. Once you have the curve, do three things: find the single worst step, since one stage usually leaks far more than the rest and that is where your attention belongs; measure time in stage alongside conversion, because a stage where deals sit for eleven weeks is a qualification problem wearing a conversion costume; and use the rates to work backwards into how much pipeline each rep needs, which turns a vague target into an activity plan.