It's actually "at most 50% on toil". But of course it's also hard to measure exactly. It's more of a barrier that if we exceed it, the team probably needs serious help!

Ultimately, Google engineers can move between teams pretty freely, so if we allow a team to descend into an operational or a deadline-induced death march, and we don't address that quickly, chances are the engineers will move to another team. It's sometimes frustrating not to have more control as a manager, but it's a very nicely self correcting mechanism and fixes various incentives for us managers.

(I'm a manager in Google SRE. Not speaking for Google.)