Courts do this stuff all the time. Meta was just fined $18B for making their services addictive, and failing on purpose to keep kids away. That number is entirely made up.

How it works is that the complainant makes an argument with an amount attached, the argument is reviewed and discussed by all parties, and the judge decides. The law system is amazingly free-form in some respects, which is why precedent is so important: there's not much else constraining things.

Generally, one side would come up with their idea of the number and state their reasoning, the other side would do the same, and the judge looks at their reasoning and decides what makes sense.

This requires a lot of work to estimate and reason and argue and adjudicate, you're not going to get a napkin math answer on an internet forum.