I didn't really think my rating system all the way through when I first started, I have mostly been coming up with the numbers by instinct. Here I shall lay out all of the vague ideas I have in my head, and I will do my best to stick with it. However, I reserve the right to rate something as higher or lower than it technically is depending on how natural or plot-necessary it is.
1. A couple of uses of made-up swearwords, or one or two uses of mild profanity, such as the word "damn" or "hell."
2. A few uses of mild profanity.
3. A single use of the word for an illegitimate son, or an impolite word for a woman, or the sh-word.
4. A few uses of the type of word listed in #3.
5. A single use of the f-word.
6. Frequent uses of mild profanity and one or two uses of a worse word.
7. Several uses of the words described in #3, plus a single use of the f-word or several uses of mild profanity.
8. A few uses of the f-word.
9. Many uses of all kinds of profanity.
10. Constant and pervasive use of strong language.
1 and 2 mean that there is practically no sexuality, only mild innuendo or a brief reference.
3. Some innuendo and multiple references.
4. A scene of sexuality that fades away before anything happens, but you still know what happened.
5. A scene that gets to the touching stage (but not the touching of certain body parts) before it fades away.
6. A scene that doesn't fade away but is still rather vague.
7. One descriptive scene.
8. Multiple scenes of varying explicitness.
9. One or two disturbingly graphic scenes.
10. Many graphic scenes throughout the novel.
Violence is a harder category to quantify. You can have a battle with no blood, and a fistfight that is brutal or vise-versa. You can also have a bloody scene that doesn't bother you in its context, or a fight with no blood that is psychologically more disturbing. For this category, I will be more general.
A significant amount of blood will earn a 4 or up.
Rape, no matter how vague, will always earn a 5 and up, as will murder.
Graphic gore, such as entrails or amputation, will get a 6 and up.
Anything above a 6 will likely be one of the above only more graphic, disturbing, and/or in larger quantities.
Any ratings that do not seem to match up with what I have listed above, for example, a rating of a 4 in sexuality for references only, means that there are more references or worse references than I would rate a 3. I hope this is useful to you in understanding my ratings, I know it will be useful to me :)