An invalid formula: "For late delivery the seller pays a penalty of 0.1% of the value of the goods." Why? Because it states neither a maximum amount nor the period over which the penalty accrues.
A valid formula: "For each calendar day of delay beyond the agreed delivery date, the seller shall pay a penalty of 0.1% of the value of the undelivered part of the goods, but not more than 10% of the total contract value. The penalty accrues until actual delivery or termination of the contract."
Mandatory: define the event that starts the clock (bill of lading date, date of arrival at the port) and the evidence of delay (CMR, acceptance certificate).