GMAT Strategy

I have 3 questions:

1. With 30 days left, is it better to focus on one source of material (Official Guide) or multiple sources of material?

I have worked all of the Official Guide problems and some Princeton Review questions pertaining to my weaknesses. My plan was to repeat the OG problems with a timer to ensure that I can get all of them completed within 1-2 minutes. Is this a waste of time? Would you recommend branching out into new materials (Manhattan GMAT, Kaplan, etc) instead of focusing on the OG?

2. How do you approximate your GMAT score?

I've noticed that people on the forum have a strong understanding of what their score is. Aside from the Official GMAT Prep software, how do you approximate your performance on the test? I have a vague understanding of my performance, but because the score is based on timing and level of difficulty (not percentage correct), I have no clue how this translates to a GMAT score.

3. Do you have any tips on building endurance and stamina?

I've been studying 3 hours per day on weekdays and 4-7 hours per day on weekends. I have found that I can usually only do 1-2 hours without getting fatigued... which usually results in a streak of wrong answers. Any tips on how to stay alert for 4 hours straight? How many GMAT problems do you typically do in one setting?

The problem with redoing OG is that you already know the questions and their solutions so this will not really test your skills on unknown problems. If you don't know the questions well, then it's a good idea to resolve them timed. Try to extract a moral from each problem, not just solve it mechanically. What is the problem testing? Is there a typical trap in it? How to recognize such traps in the future and avoid falling in them?

If you are a high scorer, perhaps it's better to get the Problem Pack I for GMATPrep and train with those or simply go to gmatclub.com and select problems in a certain area with a certain level of difficulty and solve them with the timer on-site. A good book with problems resembling the official style is Princeton Review 1037 GMAT problems.

For stamina, simply repeat GMATPrep tests. I've repeated GMATPrep more than 15 times and I still get new questions. Deinstall and reinstall it if doesn't allow you to repeat the practice tests anymore. Analyze your errors on GMATPrep and think about their causes and how to avoid them. You can also do other CATs like MGMAT or GMAT Club.

For endurance during study I would suggest taking Piracetam (sold on Amazon). It's not a stimulant, just keeps you going. Taken late at night, it may prevent you from falling asleep. On the actual exam you may consider taking a stimulant of your choice to give you stamina and speed but not jitters, Piracetam doesn't work for that.