From personal experience, the diagnostic is a great tool but I did not get many questions right when I did it and I passed the PE exam. What I would do is try to understand the material of each question. If I got the answer right, great. If not I would try to understand what the right answer is and how to do it. I once spent over 4 hours trying to understand 1 of the questions on the NCEES practice exam. By trying to understand the solution, looking up relevant material, asking questions on this board, I picked up alot of material on my journey. The final result was a familiarity of WHERE to find the solution for that questions and others similar to it.
My point, use the practice test as a learning tool more than as a diagnostic. If you did not get enough questions right, it will only scare you. It does not necessarily mean you should think about pushing off the exam.
Best of luck with your studies.