Law firms usually use an online verbal reasoning test as part of their first round of assessment after you submit a good application form.
From what I know, most firms have a pass mark that you have to meet in order to go through to the next round of assessment etc. With respect to what that pass mark is, I suspect it varies from firm to firm but from my experience its about getting as close to 75% of the questions correct as possible.
The way it usually works is that the law firm is able to see which percentile range you fall into - they receieve data for all the results of their applicants who have sat the test. They are able to distinguish between candidates on this basis.
Also, from recent feedback that I received from a large city law firm for which I had to sit a verbal reasoning test, they also are interested in seeing how many of the questions you were able to answer in the paper. There is something like 45-55 questions that you have to answer in about 30-40 mins (it varies from test-to-test and firm-to-firm). As a result, I also suspect they get to see exactly how many you got right from the number of questions you answer. I tried a strategy of concentrating on answering only 75% of the questions as I thought this would allow me more time to answer the questions correctly; the point is, i think its ok to use this strategy as long as you answer a respectable number of questions. On the other hand, its entirely up to you, these are just my experiences.