
This question is relatively easy to answer for a single law school: load up the school's website and check their "Entering Class Profile" information for the GPA and LSAT of the middle 50% of accepted applicants. This information is usually shown in this format:
Median LSAT: 167 (75th percentile: 170 / 25th percentile: 164)
Median GPA: 3.82 (75th percentile: 3.92 / 25th percentile: 3.7)
These numbers mean that an "average" applicant with that score combination was accepted (and enrolled). Obviously, being below the median lowers your chance of admission, while scoring above increases it. I couldn't bring myself to have "below the median" in red, as those scores are still impressive, highlighting how difficult it can be to be accepted to a top law school.
In the example of Boalt above, it's possible to cross reference the two scores into a target LSAT score for a student with a 3.7 GPA, such as "Diane." Being at the lower GPA bound of the average admitted pool, it would be best for Diane to score at least a 170 on the LSAT to solidly fit within Boalt's acceptance range. Conversely, a student with a stronger GPA of 3.92, "James," at the higher bound of the average admitted pool could score a 164 or above and still have a strong chance for being accepted. Basically, criss-cross reference where your GPA falls within the median to find your target LSAT score.
Here is a graph comparing the predicted acceptance rate for each fictional student, Diane and James, with their LSAT score as the independent variable. The data are taken from the law school calculator, which has a link on the toolbar to the right under "Zen + These = 99th Percentile." It's pretty self-explanatory, but we'll have our two current students walk you through their own law school calculations as part of a goal setting exercise in video posts later this week.

However, all other factors being equal, these numbers form a basic answer for the questions above: "I have a 3.81 GPA--the middle of Boalt's GPA--so I should score at least a 167--the middle of Boalt's LSAT.
The question then becomes: "What if I don't have a GPA that's within the median range?"
If your GPA is above the 75th percentile, then you're sitting pretty and should shoot for at least the 25th percentile LSAT score. If your GPA is below, especially significantly below, then you'll need to resort to more carefully calibrated tools, which we'll discuss in a later post.
And the answer to the question at the beginning of the post: 180. Get your Zen on.