I think you have nothing to worry about

I was scoring the same grades as you. Actually my highest on the Question Trainer was a 63, and my lowest was a 55. I took NCLEX-RN July 13th and still passed (first attempt) with 75 questions.

As long as you're getting above 50% and reading rationales to wrong and correct answers, you're all set

Also look at how you're doing on the analysis and application section (Bloom's Taxonomy for Question Trainers). If you're scoring above 50% on those, you'll be alright. Good luck with the studying