Definitely DO NOT think twice about your Kaplan diagnostic scores.
I swear to you, I think I got 12 on their diagnostic test...3 months later I scored well above the 30 I was shooting for, and was accepted to every school to which I'd applied. SO (based on anecdotal evidence

...) DEFINITELY do not read too much into the diagnostic. You will learn TONS throughout the course and get better with each test.
BEST OF LUCK!