We do not publish 50th percentile ranges for SAT II subject exams since they are not a required part of the application and not all applicants submit those scores. We do recommend SAT II subject exams, as stated on our standardized test web site:
http://apply.jhu.edu/apply/testreqs.html.
As far as how are they evaluated, they are looked at as just another piece of academic information for an applicant. They are in no way the most important piece of information we receive about an applicant, but if they are included within the application they do matter. In the academic review of an applicant, the thorough evaluation of one's high school transcript will always carry more weight than the review of standardized test scores.