I like the dedication on this thread, even if you are barking up the incorrect, but infinitely tall, tree. Here is my take on it (and I am in no way affiliated with any of the GMAT people)
To the best of anyone's understanding, there are three major factors that go into a score:
1. # of questions answered
2a. # of questions answered correctly
2b. Difficulty of the questions answered correctly
While the GMAT will never penalize anyone for answering a question incorrectly, it cries when you run out of time prior to finishing. Leaving the last 3 questions blank will probably knock off 3-4 points off your final score. Getting the last 3 completely wrong may not affect anything at all (and there's always a chance that you will accidentally get one right).
Invariably, you're going to get some questions correct. The GMAT takes these questions, and runs it through its immense database of how well other test-takers have answered that particular question, and what their final scores were. Here's an example: question X was answered correctly by 50% of people who scored a 50 in Quant. That makes question X a level-50 question. IF you get more than half of your level-50 questions correct, you get a Q50 (all other things being equal).
Of course, all other things are rarely equal. To be on the receiving end of a high number of difficult questions, you must first answer the simple questions correctly. If you're not getting 70+ % on the easy stuff, you're not going to get the super-hard stuff. The GMAT's algorithm (whose intricacies and hobbies we mere mortals will never know) assesses the collection of questions you've answered correctly and decides whether or not you're worth a more difficult question. If you're inconsistent in obtaining the correct answer at a given level, you're going to be stuck there until the program has ascertained that your right answers are not flukes, but rather the result of an accurate assessment of the question prompt.
What's more - the exam tends to observe your performance on various subtopics. So if you're a super voodoo wizard at geometry but useless at every other part of Quant, you won't get very far, even if you do somehow manage to receive a disproportionate number of geometry-related questions.
...all of this mess, which may or may not be accurate, brings us back to two questions that were asked:
- How do you get a V42? By being in the top 4% of Verbal test-takers. There's no formula, and it is impossible to know.
- How is the grading basically obtained? It is not basically obtained, unfortunately. Rather, it is complexly obtained.
I apologize for being obtuse about these matters, but I have found that the sooner you become inclined to forget all about the GMAT scoring intricacies, the better off you'll be. Go out, buy OG13, do the diagnostic, and get to work. For most of us, there is no other way.