The JavaRanch mock exam is a great learning tool, but I think it's easier than the real exam and isn't a good measure of how ready you are.
The HFSJ mock exam is a much better indicator. You shouldn't take it until you've thoroughly studied HFSJ and you think you are ready for the real exam. In my opinion, if you score 75% or better you are ready to sit for the exam.
If you don't get 75% or better, resume studying. Don't return to the mock exam (and don't study the questions) until you are sure you are ready. If you take the mock exam too many times, you'll start to memorize the answers.
[ March 24, 2006: Message edited by: Scott Johnson ]