This is a good place to ask without a top-level comment: Do people really tend to do it that way? Whenever I've considered using sleep to mitigate timing attacks, it has been to sleep an amount of time that makes the resultant time constant. That is, to make the operation in this case always take 450ms.
Sleeping just long enough to fill the remaining time-slot for an operation is actually a valid countermeasure; in theory.
The problem is figuring out how much sleep is necessary based on what variable time work has been done, and actually sleeping the right amount. This can be difficult with the granularity involved.
In practice, attempting to fill the remaining time is either not effective, or requires a level of instrumentation that severely degrades performances.
