Each school weighs them differently, so I can’t really give you a flat percentage or anything. If your grades are great but your LSAT is mediocre, the admissions folks might think that you have a good work ethic but aren’t the sharpest knife in the drawer. If your LSAT is great but you have mediocre grades, they’ll likely think you have a sharp mind but tend to blow off schoolwork. Low grades are, I think, easier to explain and get by with than a low LSAT; I explained mine in a paragraph of my essay by relating the story of my last semester as an Electrical Engineering major. Because my grades were good in my degree subjects and because the class that killed my grades was Differential Equations (advanced math class totally unrelated to my eventual degree), I recalculated my GPA to show a more accurate portrayal of my abilities and included that in my essay.
Which brings me to another point: your essay is the one thing that will distinguish you from other applicants of equal ability. If you’re a borderline candidate on scores alone, the essay can make or break you. Take it very seriously, write many drafts, and work on it over a period of months. Books on graduate or law essays will have additional specific advice.
As someone noted earlier, a good lawyer must have good communication skills, so writing ability is a major plus. In case you didn’t know, one portion of the LSAT is a writing sample, usually just a few paragraphs addressing a hypothetical situation in about 30 minutes. It’s not scored, but a copy of your writing sample is sent to every school to which you send your LSAT scores. Remember those techniques that you learned in high school English, and use them well.