People who really understand Lean understand that slack time is essential. But you ask how to convince a manager of that. Well, perhaps you can send your manager into a traffic jam a few times and see if you can convince them that highway traffic jams are exactly the same thing that happens in a development organization that tries to drive utilization to 100%. There is enormous waste generated by such practices.
I think you need to start by coming to an agreement that cycle time - the time from customer request to request satisfied - is the best measurement of the performance of an organization. This is not an easy task, but once you are there, slack time will be built into the organization.
See chapter 5 of the book, which is all about this very concept.
Author of Lean Software Development, Implementing Lean Software Development, and Leading Lean Software Development