Congratulations! My diagnostic was even lowerthan yours, so I absolutely know how you felt about your first test score. What schools are you looking into, if you don't mind me asking you that.
I would say for me that my score with newer tests (in the 80's) was 1-2 points lowerthan my score with the 60's preptests. But you are still weeks away and I actually improved a lot in the last 3 weeks of studying (like 2 points).