The real problem.
An apparently correct form can hide errors in amounts, periods and labour rules. The interface needs a model that preserves what was calculated, from which data and at what point in time.
The architecture decision.
Treat calculation as a verifiable domain independent of presentation. Each material correction to the prototype comes with a regression test; closed periods are frozen to prevent retroactive billing changes.
- 01Period data
- 02Calculation rules
- 03Review and close
- 04Immutable audit log
The controls that support it.
- 232 tests over the calculation layer, across eleven suites according to the CV.
- A 90% coverage threshold for integrating changes.
- An audit log that the database prevents editing or deleting.
- HTTP forms usable without JavaScript, verified with fifteen end-to-end tests.
Evidence with context.
The CV documents nineteen screens, sixteen tables and 42 architecture decisions with their rationale and rejected alternative. The system includes monthly per-business-unit billing and frozen closed periods.
Source for figures and scope: the author’s 2026 CV. This is not live telemetry.
The limits matter too.
Coverage measures which lines were exercised, not the universal correctness of a rule or calculation. Maintenance must follow regulatory changes and retain regression cases. This page does not calculate payroll or provide employment advice.