Here is another example of a simple error of omission that could have been caught if the student had read the essay aloud or given it to a friend to read. The word "of" should be between "calculation" and "the." That one small error makes the entire sentence awkward and confusing. If the instructor has to reread the sentence to try to understand its meaning, the flow of the essay is interrupted. If this happens often enough in the essay, it gives an overall bad impression on what otherwise might be a very good paper in terms of research.
It really depends on a number of factors, one of which is your understanding of the test and requirements of the band scores. If your English is suitable for the band score you are aiming for, you will only need to focus on exam skills to make sure you reach your potential. The skills will help you understand how to approach questions and what the examiner is looking for in your writing and speaking. Once you feel comfortable with all that, you will have a clear idea of how long you want to practice before your test. Whatever you decide, make sure you are frequently getting the band score you want in practice tests.
All the best
One option is to organize your essay from general to particular . For example, if you were describing the new Big Tex at the State Fair of Texas, you might start out by describing the setting - the smell of the corn dogs frying, the crowds of people, the happy children dodging in and out of the midway games - then get into his grandiose size - after all, everything is bigger in Texas, right? After that, you describe the details: his new crisp, white, pearl snap shirt, his blue jeans tucked into cowboy boots, adorned up top with his signature belt buckle, and so on.