I just took the PMP Yesterday, I can only give you my experience. The way the question should be worded, is when do I know I am ready for the exam? I think that's a question all people taking the PMP struggle with. I approached it based on consistency in all areas, I was aiming for +75% in Mocks. In the end for Precast simulator I got 70% first exam and then 77% in the second I switched to PMStudy Hall and got 77% in the first ( shows Precast works!) then then 73% in the second but I messed up the timing as it was in hours not mins and % not question count down and I got confused, so I think I could have got higher.
I decided if I was getting mid to upper 70%s in the mocks and with no major weak spots I was going to go for the exam, I was suffering exam fatigue and reaching diminishing returns, also 4 hours for each exam is a significant investment in time and to get the best value out of them, treat them as a real exam. So I felt I was burning myself out to do more exams and I was getting also 80% in the quizes. On each mock I set myself a target and I spent a lot of time reviewing wrong questions and then going back to the books to brush up, here I have to say the prepcast simulator is excellent, better than PMI Study Hall as I was really able to root cause my reasons for wrong answers and iron out my weak spots.
Today PMI notified me I passed the exam, now this cannot be a guarantee that +70% you will pass, in fact there is no certainty, you just have to make your own judgement on what percentage on mocks you are willing to accept to do the exam, as a very general rule of thumb below 70% is a higher risk and within 75% to 80% in mocks is what a lot of people aim for, however I stress even if you get good marks on the mocks the key is also do quizzes in each domain and ensure you do not have any significant weak spots. It is consistency in all areas that's the key.
Personally for me I was very happy to set myself the 75% on Mocks as my target for readiness, I knew I would never get to know the PMP actually Percentage, I couldn't control it so I didn't worry about it. So based on my own research I set a mock target and for me it worked out. I understand it can be quite stressful and believe me I understand, as I have just gone through it! Best of luck with the exam!