I found what has been said about the GMATPrep being the most accurate indicator to be true. So if you scored that in your GMATPrep, it is very likely you'll get that on the real thing (sorry I know that's not what you want to hear). In addition, I dont think you can ever 'safely add 100 points' to your practice score. That's just a bad idea to even think that way. You may score 100 points higher depending how well you perform under the gun, but I would never ever bank on such an idea. That's a recipe for disappointment. For me personally, in the back of my mind, I know I usually perform better under pressure, but I would never prepare under the assumption that trend would continue. If anything, I always would err on the side of caution, and judge my potential real score as being lower than a practice score ... If you're shooting for a 640, then you should be hitting that or very very close to it on practice runs.
My advice is to review how you're studying. Without knowing what you're doing exactly, I can only tell you my experience. In a broad sense, there are only two levels of preparing that you need to take. One is the content, the other is test taking. My approach was to treat them separately, with the test taking coming after I felt I had a decent grasp of the basics.
Do you feel you have a decent level of understanding of the principles that are tested (number properties, geometry etc..)? If yes, then it's just a matter of practicing test taking, which is the second step. If no, then you're trying to take practice tests without knowing the prinicples, which is going to lead to crappy scores, and varied ones at that, because your score may drop based on the types of questions you get on each trial. You may tested more on stuff you haven't studied on test 2 while test 1 may have had more things you're familar with. Until you covered all basis, then you may run into big fluctuations.
If you have the material down, then the second step is learning to apply what you learned into answering the test questions. My first practice exam was a 600 (this was after I felt I had learned the basics already). I just took a bunch of practice tests over a month period and tried to identify weaknesses, and was able to score in the high 600's , hitting 700 once in practice runs. I scored a 700 on the real thing, so I basically got my score up 100 points on practicing test taking alone.
Point is, if content is your problem, i'd focus on that instead of practice tests. I wouldn't be concerned with the score, because it's really an incomplete score as you haven't completed your studies. If you have content down, then it's a matter of taking more practice tests and trying to learn from them.