National tariff calculations lived in fragile Excel files: multi-sheet workbooks with cross-referencing formulas that only their authors could maintain. The Ministry needed the same mental model — sheets and formulas — as a proper system.

I designed and delivered the system end-to-end using AI-assisted development, translating the analysts' Excel workflow into a web product without breaking their habits.

The heart is a recursive calculation engine with cross-sheet REF formulas — implemented twice, in Python and TypeScript, with verified parity between the two. A complete multi-sheet template ships with a 9-appendix heating-tariff dataset as the seed.

• 2×calc engine parity — Python & TypeScript
• 9appendices in the production tariff seed
• Livedeployed to production infrastructure