The rule is in the model, the number is not

Article 7.4.9 names how the rate of interest is chosen, not what it is. A model of the instrument that invented a percentage would be worse than one that says where the number has to come from.

La conversation a été enregistrée en anglais ; les répliques sont reproduites telles quelles.

The buyer is three months late paying an invoice denominated in euros, payable in Frankfurt. What interest are we entitled to under the UNIDROIT Principles, and at what rate?

Two questions, and they are answered by different halves of article 7.4.9. Let me find the addresses first.

Entitlement is 7.4.9(1); the rate is 7.4.9(2), and the model carries it as three predicates rather than a number — the rate at the place for payment, the rate in the State of the currency, and the rate fixed by that State's law. Entitlement first.

Established, by a strict rule, with the excuse question expressly irrelevant — that is 7.4.9(1) working exactly as written. Look at the second line, though: the same run also derived the THIRD tier of 7.4.9(2), the rate fixed by the law of the State of the currency. That is the tiers doing their job on the facts I gave — I said nothing about any bank rate existing anywhere, and the third tier is the one that applies when the first two are not known. Your case does have a market fact: you pay in euros in Frankfurt.

So the governing rate is the average short-term bank lending rate to prime borrowers for euros at the place for payment. What the model will not give you is the percentage — and that is not something I should work around. Let me ask for the number the way a user would, so the boundary is on the record rather than in my paraphrase.

There is no such norm, and inventing one would be the worst possible service: a number that looks as authoritative as the rest of the answer but came from nowhere. The Principles themselves name the rule of choice and leave the figure to evidence — the actual bank rate for euros in Frankfurt over your three months, which comes from a bank, not from a rule. The model has followed the instrument exactly that far and stopped where it stops.