Part-time at Deutsche Bahn DB?

I am parked at DB Fernverkehr.

Now I'm considering going part-time.

I was thinking 80%.

Now someone told me that it is important to apply for at least 81/82(?)%, because otherwise my benefits such as pension and vacation will be reduced proportionally more.

Does anyone know anything about this? Is this really the case? What else should I look out for?

Thank you 😊