A back-of-the-envelope round where the envelope has a purchase order attached. The arithmetic is the easy half. The scoring is in which assumptions you write down, which way you round, and what you say about the number being wrong.
Updated Aug 2026 · Grounded in real Forward Deployed Engineer interview loops and written to a senior-engineer editorial bar.
A back-of-the-envelope round where the envelope has a purchase order attached. The arithmetic is the easy half. The scoring is in which assumptions you write down, which way you round, and what you say about the number being wrong.
This is estimation with consequences, which is the version FDEs actually face and the version most candidates have not practised. The senior signal is stating assumptions as named variables the customer can challenge, rather than producing a single confident number. The reserve probe is the direction of error: over-provisioning is recoverable and reads as waste at renewal, under-provisioning is an outage plus a second procurement cycle, and a candidate who has an opinion on which they prefer and why has been in this conversation before. Watch for anyone who computes a peak number and does not ask what the peak is, or who gives a point estimate with no band.
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