The conversion is done differently for each version of the test.
My DD did far better than I'd anticipated, based on the sample questions. She had no interest in even looking at the sample questions, though, and is generally diligent in an actual test environment.
I will say that her kid-perception did not map to actual test results. One section, she answered all 30 questions, said she thought she got them all right, and scored a 19. One section, she answered 22 of 28, said she guessed on almost all of them, and scored a 19.
