Look up the concept of "day fines", which are used in Finland elsewhere:

https://en.wikipedia.org/wiki/Day-fine

Surprisingly they were experimented with in the UK, for a very brief period of time. But not taken into use.

Every now and again a particularly large fine, often for speeding, will make the news. For example this story does the rounds now and again "Finland, Home of the $103,000 Speeding Ticket":

https://www.theatlantic.com/business/archive/2015/03/finland...

That doesn't answer the question, do they work better than flat fines and points on licenses (that lead to suspensions)?

How do you decide if something like that works? Is it by stats on "repeat offenders", or something else?

All I can say is that when you've been asked to pay €50,000 for speeding I suspect you'll be hesitant to speed again, and so I believe the system works.

So the answer to the original question is that your confidence was based on feels.