What is generally an acceptable level of candidates to pass on to my boss (he currently manages about 15 people)?
It is not about the percentage (or fixed number), but rather about the quality of the candidates.
In my case, I like to use two criteria:
Does the candidate meet the absolute minimum for the role?
If so, then I provide an evaluation (with some sort of ranking, as in "borderline", "fair", "good", "very good", "outstanding").
The reason for the first is that you don't want to be forced to pass candidates that would be unable to do the job. If because of this no candidate goes to the next phase, then there is a problem in the recruiting pipeline (HR, external sourcing...)
The second then lets me do a honest evaluation, gives enough information for my manager to take a decision (possibly combining my feedback with others' evaluations), and, if necessary, provides my manager with a way of ranking the candidates (so, if he wishes, he can just take the top N from the list).
Other than that, I usually refuse to trim the list of candidates to a fixed number - as there is the risk of having me as the single person that filters out too many people.