Yes, netzpolitik.org has some info [0], I'll give you a deepl translation of the section:
In the future, performance not in compliance with the contract will be deemed to have occurred if
- 90 percent of the contractually agreed maximum speed is not achieved at least once on each of at least two measurement days, or
- the normally available speed is not achieved in 90 percent of the measurements, or
- the contractually agreed minimum speed is not reached on at least two measurement days respectively.
To be recognized as valid, users must also perform at least 20 measurements on two different days and have their computer connected to the Internet via LAN.
[0]: https://netzpolitik.org/2017/weise-deinem-netzanbieter-nach-...