The tests cease to be useful when your score hits a plateau and there is no noticeable weakness or gap in your preparation.
Therefore, the tests are useful as long as they help you zero in your weak areas. Additionally, the prep tests will help you develop the stamina for the actual test.
I didn't really understand what you meant by "track my preparation." Could you please clarify what you meant?
STUDY PLAN
Week 1
1. Write the GMAT Prep Test 1, downloaded from mba.com. The scores will tell you your relative proficiency in verbal and quant.
2. Pick 3 of the weakest/least comfortable chapters (such as geometry, algebra, number properties etc) in quant. Study these topics only for the rest of the week.
3. Pick the weakest/least comfortable question format (SC or CR or RC) in verbal. Work on that question format alone for the rest of the week.
Week 2
1. Write the
Manhattan Prep Test 1.
2. Pick 3 of the weakest/least comfortable chapters (such as geometry, algebra, number properties etc) in quant. Study these topics only for the rest of the week.
3. Pick the weakest/least comfortable question format (SC or CR or RC) in verbal. Work on that question format alone for the rest of the week.
Week 3
1. Write the
Manhattan Prep Test 2.
2. Pick 3 of the weakest/least comfortable chapters (such as geometry, algebra, number properties etc) in quant. Study these topics only for the rest of the week.
3. Pick the weakest/least comfortable question format (SC or CR or RC) in verbal. Work on that question format alone for the rest of the week.
Repeat procedure till your score saturates and you no longer have any areas to strengthen.
Final Week.
Write GMAT Prep Test 2, downloaded from mba.com
This score will give you the most accurate projection of what your score might be if you were to take the actual GMAT within the next 2 weeks.
CAUTION
To get the best result, the prep test environment should be close to the actual GMAT test center. For this,
1. Make sure you're not disturbed during the entire test duration.
2. Make sure you time your breaks (8 minutes between Quant and Verbal)
3. Finish the entire test no matter how distracted you feel.
4. Write the AWA (only for the last 3 prep tests)