The Manhattan practice CATs can be considered "accurate", but honestly, I only know about their accuracy into the higher ranges of scores. I would recommend that you take the FREE GMATprep test from MBA.com because it will absolutely give you an accurate idea of how you're doing. Also, I would recommend that the books you purchase or study with are all about the basics. With just the basics in hand, you will be able to get that 400+ that you need. You will not have to be killing the toughest problems the GMAT has to offer. In fact, I would recommend that you skip the crazy tough ones and make sure to spend the time and get the questions YOU CAN get right. Best of luck to you!