A couple things:

First, I think that hoping for a 50 point increase in one week might be a little optimistic. As a baseline, the average increase for Veritas students over a seven-week class (plus a couple more weeks of study time) is about 100 points. It's certainly possible you'd make up 50 in a week, but more likely is that your score wouldn't change much.

Also, students tend to do roughly the same on the actual test as they did on the most recent practice tests. So, given your practice scores, I'd expect you to get around 630 or 640. If that's good enough for you, there's probably no harm in taking the test. However, if you're shooting for 700, you might want to take some more time to study and wait until you're hitting 700 on practice tests.

Good luck!

Thanks Jim! I deferred it now...

In the meantime I went on studying. I did another couple of practice tests...
and I still have "huge" discrepancies in my scoring. So sometimes I would have a 600 - sometimes a 680 ... I noticed that I have some problems in switching from PS to DS during the tests and still sometimes I am quite weak in SC.

Do you have any good hint how I can get more "stable" in my scoring... ?

Thanks in advance, Lisa

It's not surprising to see big variations. For one, every practice test is a little different, and at best they are approximations of the real exam. So, I'd probably say to split the difference between those two scores and expect a 640.

The other consideration is that the actual GMAT has a margin of error of 30 or 40 points. So, you might get anywhere between a 600 and 680 if your "true score" is a 640. A lot depends on luck, silly mistakes, and the batch of questions you get.

So, I wouldn't be alarmed - just keep studying.