Here are some indications of how I, Adam Elga, personally understand various representative percentages for independent work.
(Adapted from Jim Pryor's guidelines on grades, with some language from Princeton grading documents.)
Significantly exceeds the highest expectations for undergraduate work. This thesis is either a practically flawless execution of an exceedingly ambitious project, or else makes a contribution that is incredibly original for an undergraduate. (As a baseline, I can report that having advised senior theses at Princeton since 2000, I have yet to give a grade in this range.)
Meets the highest standards for the assignment. The thesis is clearly written, cohesive, well-argued, and original. A thesis that gives a straightforward response to some philosophical or interpretative problem would not merit a 95%, even if it is extremely clear. The main argument of a 95% thesis has no obvious flaws, and has an interesting conclusion. A 95% thesis does something extra--but not at the cost of a clear treatment of the problem.
Meets very high standards for the assignment. This thesis operates at an advanced level. It is ambitious and clearly written. If there are any significant problems with the writing or the organization of the thesis, then it won't merit a 91%. This is because good clear writing and organization are not separable from good philosophical thinking. The thesis may have a couple of minor mistakes or confusions, or it may fail to unpack some of its arguments sufficiently. Perhaps it includes a small amount of irrelevant discussion. The thesis may have an intriguing original argument or interpretation, but if so, that will be offset by some other flaw. For example, perhaps there is too little philosophical back-and-forth (considering objections and challenges, and responding to them). Or perhaps the thesis is not as engaged with the texts as it should be. On the other hand, the thesis may not have such flaws, but if so, that will be offset by its having a less original or ambitious main argument.
Meets high standards for the assignment. This is a well-written thesis with nothing terribly wrong. The writing may have some small problems, or it may be flawless. The thesis may make some mistakes or have some ambiguities that have to be sorted out, but overall it will be a very good paper. An 89% thesis has a clearly marked main argument, which has some plausibility and interest. A thesis that just makes routine observations about a topic and spells out a fairly clear but straightforward argument, won't earn an 89%. It will show more promise or originality than an 85% paper, but nothing will make it stand out like a 91% paper, or it won't be operating at as advanced a level as an 91% paper. (Or perhaps the thesis would stand out if some of its ideas were fully developed, but as it stands they aren't.)
Meets most of the standards for the assignment. This is a good thesis overall. It contains some notable mistakes, unanswered objections, or obscurities, but no serious misunderstandings. The writing may not be super-clear. To earn an 85%, the thesis needs to make it clear why the problem it addresses is a problem, and offer some response to it. (It may be a straightforward or unoriginal response; it may not be a decisive response; the thesis may even end by showing that a certain response doesn't work. But the thesis must put forward or examine some response to the problem.) An 85% thesis does not seriously misrepresent the views of other philosophers.
Shows some reasonable command of the material, though there are moderately serious problems. Perhaps the writing is unclear or the paper is poorly organized. Or perhaps there are straightforward mistakes or misunderstandings about what the problem is, or about what other philosophers say. Or perhaps the paper presents the problem correctly, but doesn't really address it. Still, there is significant effort. The author has some understanding of the problem and of the relevant texts. The author does offer some argument. A thesis with no argument won't merit an 81%.
Meets basic standards for the assignment, but there are serious problems. Either the writing is really hard to get through; or the thesis has little discernible structure; or the author doesn't understand the text or the positions she is discussing; or the thesis doesn't really attempt to offer any argument. Alternatively, a 79% thesis might be written in polished prose but contain ideas that are obscure or cryptic. A thesis giving the sense that the author didn't put in much effort won't ordinarily earn a 79%.
Papers with more problems will earn grades of 75% or below. It is difficult to give a general gloss on those grades since the problems that beset these papers are quite varied.
Meets some of the basic standards for the assignment.
Falls short of meeting basic standards in several ways.