There is no simple answer to how to strike a "perfect balance" -- after all, this is in many ways the primary challenge of the test! Also, on some level, it differs for every test-taker. However, in my experience, here are the things to keep in mind and try out for yourself:
1) The most important thing -- by far -- is to finish the test. So, if you have a few questions left but only 30 seconds remaining, it is essential to click click click and at least have random guesses in for all 37 Quant questions (and for all 41 Verbal questions). Every question left blank will drop your scaled score by 1 point. Really.
2) If it's a matter of only 3 or 4 questions at the end of the section, then honestly you don't have much to worry about; you'll get slightly faster as you continue to study, and if you have to guess the last couple on test day, that's okay.
3) Part of an adaptive test is getting questions wrong. Everyone gets SOME wrong, it's inevitable. It is possible to score 800 and still get something wrong. So, if you're running out of time regularly, think of this: If you're not going to get to answer 5 questions (at the end), then instead think of it as having 5 free "skips" in the middle of the test. When you come across a problem that looks particularly challenging or bizarre, don't even bother; spend 30 seconds eliminating answers and guess and move on. On my GMAT in December of 2009, I intentionally guessed on 3 questions in the middle of the test because they seemed very difficult, and I was very satisfied with my 50 on the Quant section. If you too would be satisfied with a 50 (99th %ile), then you can do this, too.
4) Never try to "catch up" by rushing through questions. As you learned your first time, if you try to do questions too fast, you will end up just getting them wrong. On each question, decide early on: "Am I going to actually do this? Or am I going to give up and guess?" And after 2 minutes, if you haven't reached an answer, ask the same question again. Never rush: either do the question the right way, or don't do it at all.
5) In general, timing is about predicting where the test is going. As you're studying, categorize questions. The best test-takers approach each question not from scratch, but from a position of recognition. We say, "Oh, I've seen this one before, I'm going to do the same thing I did last time." This really speeds up the beginning of the problem. However, you can't reach this level overnight; it requires practice and practice and study to the point of a pretty unhealthy familiarity with all of the topics on the test. When you've reached this point, and you're confident and aggressive enough to attack each question like the ones you've seen before, your timing will improve.
1) You must finish!
2) It's okay to have to guess on a couple.
3) It's okay to give up and guess on a couple in the middle of the test.
4) Never rush!
5) The more familiar with the questions you are, the more aggressively you can attack them and set them up faster.
Hope this helps!
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