The governing equation
Available budget = minimum Tx power − receiver sensitivity − system penalties. Consumed budget = Σ fiber loss (worst-case dB/km × length, at the worst wavelength) + connectors (0.5–0.75 dB per mated pair, per spec) + splices (0.1 dB design) + splitters + WDM/filter elements + margin (2–3 dB lifetime: ageing, repairs, temperature). Design passes only if consumed ≤ available for the *worst* path.
Worst-case discipline
Use vendor minimum Tx and end-of-life sensitivity, not typicals. Include repair-splice allowance per km on outside plant. On PON, compute the farthest customer for reach and the nearest for overload; both must pass. On WDM, run the power budget per channel plus the OSNR budget in parallel.
The margin argument you will have
Project managers see margin as fat to cut. It is the funded lifetime of the plant: every future storm repair adds splices; every ONT swap adds connector cycles; fiber ages ~0.003 dB/km·yr. Removing margin converts tomorrow's routine repairs into service-affecting redesigns. Put the margin policy in writing and defend it once, centrally.
Verification closes the loop
Acceptance tests compare *measured* against *the same spreadsheet* used in design. The budget document is versioned, and post-repair measurements append to it — a living record from design through decades of operation.