I've always come to understand.
.910 - Decent/Ok
.920- Pretty good
.930 - Above Avg, borderline elite
It depends so much on era.
But since the lockout statistically, for starters:
05-06:
Decent/ok (average starter): .900 sv%
Pretty good (above average): .905 sv%
Borderline elite (top 10): .910 sv%
06-09:
Decent/ok (average starter): .910 sv%
Pretty good (above average): .914 sv%
Borderline elite (top 10): .917 sv%
09-14:
Decent/ok (average starter): .915 sv%
Pretty good (above average): .917 sv%
Borderline elite (top 10): .920 sv%
14-16:
Decent/ok (average starter): .918 sv%
Pretty good (above average): .920 sv%
Borderline elite (top 10): .922 sv%
16-18:
Decent/ok (average starter): .915 sv%
Pretty good (above average): .917 sv%
Borderline elite (top 10): .920 sv%
18-19:
Decent/ok (average starter): .914 sv%
Pretty good (above average): .916 sv%
Borderline elite (top 10): .918 sv%