This is a difficult question - I think GMAT Prep is the most accurate indicator. However, I would not feel good or bad about any practice test scores.
I would use challenge tests or any other practice tests as a way to improve my skills, monitor my progress and cut down on errors. Use the tests to establish some consistency - in the way you study, in the way you take a test and in the way you solve a problem. This will help you cut back on errors and maintain a consistent temperament throughout the test. This also means taking the challenge test at the same time that you plan to take the real test.