This is based purely on my experience and from experiences of other people that I observed throughout the forums. I was struggling with this issues before my test so just wanted to share my experience with others.
1. GMATprep (deviation +/- 30)
GMATprep from mba.com is the best indicator of the test. It measures your real GMAT strength. So for example if you score 700 your real score should be in the range of 670-730. However, if you see repetitive question that is going to spoil the predicition's accuracy.
If your target score is 670 and you scored 700 in GMATprep that means you have reached the safety level you need. You might even score 730!
2. Mental preparation (+/- 130)
Whenever you reach the score level on GMATpreps, the game is not over yet.
Now, you have to deal with the test anxiety and testing center conditions. Test nervousness usually means that you think what is going to happen if you don't do well on the test or even if you do - are you still going to get in before the deadline and million other questions. If you happen to unconsciously worry about the test impact on your future - you probably won't get enough sleep the night before the test. (deduct up to 30 points)
Best advice here is to prepare and take the test months before the deadline but that is also hard because we humans tend to take things seriously when there is no much time left.
The deviation here is huge. Assuming that you have taken ur GMATpreps in ideal conditions (not excess anxiety, no construction work noise, no other test takers coming in and out every 5 seconds, etc) you probably did the test in the best possible conditions.
Unfortunately, test centers usually suck and you have to take some noise which can blow your concetration in pieces; the person sitting next to you can happen to be coughing constantly or the center's hallway can be in the middle of construction. Also, you can get lousy markers and have to call and wait for proctors to bring you the replacement which can all make you very mad and result in lower score. (deduct up to 100 points)
So, even though your real strength is let say 700 under ideal conditions, your real test score can happen to be
as low as 540 but also as high as 730.
The one who flies is worthy. The one who is worthy flies. The one who doesn't fly isn't worthy