Kristoff,
If you purchased an exam simulator with only 260-ish questions, it is woefully under-powered. You will soon run in seeing the same question over and over again as you study with the simulator. As you see repeats, you will instinctively choose the correct answer if you remember what it was but may have not fully learned the concepts behind the question. The PrepCast simulator has over 2000 questions and when I used it extensively as I prepared for the exam, I rarely saw repeated questions in a 4 month preparation window.
The "Passing Score" from the actual PMP exam is a closely guarded secret at PMI. Since the introduction of dynamic testing several years ago, the passing score actually changes somewhat exam-to-exam. For example if you were sitting next to a person also taking the PMP exam, his/her passing score may be slightly different than yours as they will not have the same exam as you AND his/her questions may be weighted slightly differently.
In general: Using the PrepCast simulator, if you are consistently hitting 80%+ on full exams, then you are generally well prepared for the real exam. But there are no guarantees here.
Good luck.