Almost every time I teach a TOEFL session, someone asks me to predict their score. It seems like a reasonable request—after all, it seems like your entire academic and professional future is depending on this one number between 0 and 120, so it’s natural to want to know how sure you can be of receiving the score you need. Unfortunately, the matter isn’t quite as simple as it seems. First of all, your score on the test will depend not only on your performance, but also on the particular form of the test you happen to get (this is called equating and is done based on an algorithm that accounts for the fact that some tests are slightly harder or easier than others).
Second of all, even a totally authentic practice test can’t account for unpredictable factors like unexpected distractions in the test center, a bad night’s sleep, or happening to get test questions that emphasize your strengths or weaknesses. Although the TOEFL is standardized, no one test can be a perfect measure of your academic English ability. Also, it’s important to remember that academic English and conversational English are different skills, so your ability to speak English and thrive in social situations is a lousy indicator of your success on the TOEFL.
I recommend that you take at least two full-length practice tests: one at the beginning of your TOEFL preparations, which will establish a baseline score and serve as a good introduction to the structure of the test. Then, a week or two before you plan to take the test (or at the end of any TOEFL course/tutoring you may be taking), take another one to see if you’re ready. Your score may fluctuate a few points either way on test day—if you get a 94 on a practice test, that doesn’t mean that you’re guaranteed to score the 90 you may need, and it doesn’t mean you’re doomed if you need a 96.
I think that test prediction should only be used in a very restricted manner to decide whether you’re making the progress you need. Other than the two practice tests above, I don’t worry about scores—I worry about performance and progress, and I recommend that my students try to do the same. Try to set realistic, non-numeric goals for yourself, and you’ll find that your sessions are not only more productive (because you’re not wasting time looking at score charts), but also more motivating, since a number never tells the whole story and may not show you how much progress you’re making.