The brief was simple to state: replace handwritten revenue and expense logs with something that calculates profit and loss automatically. The hard part wasn't the calculation — that's arithmetic. It was building something the client would actually use every day without training.
The temptation on a finance tool is to add structure: categories, tags, multi-currency support, export formats, permission levels. Every one of those is a legitimate feature. Almost none of them were needed for this client on day one, and every one of them would have added a decision the client had to make before they could log an entry.
So the interface was built around a single loop — log an entry, watch the number move — and everything else was cut or deferred until there was a real reason to add it back. Profit and loss is calculated continuously rather than at month end, which changed the tool from a record-keeping chore into something the client could glance at any time.
The result wasn't a technically impressive system. It was a system that eliminated handwritten logging entirely and gave the client their time back every single month. That's usually the actual brief, even when the stated brief is about a calculation.