Like OP mentioned, the efficiency of recovery varies by speed but is generally in the 65%-80% range for total round-trip (the system captures ~80% of the kinetic energy but with round-trip losses, you only get ~80% of that energy to propel the car).

Maybe you're thinking of the total range boost provided by regen braking? That's often shown to be roughly 20% via studies;

https://sci-hub.se/10.1109/vppc.2011.6043109

> Simulations show that the energy reduction of the vehicles under test can be more than 20% by applying regenerative braking.

It's an older study, but they show the regen efficiency at 60% for the EcoTruck and 70% for the bus.