I’ve been asked questions like this numerous times. The person asking wants a black or white ruling, but the “correct” usage is often very gray. And very often, there is no “rule” but rather a choice to make, either on the part of the author or the editor.
The Chicago Manual of Style has a Q&A section each month in its online version of the guide. This is from the April 2013 Q&A section:
Q. Where in the manual will I find guidance to answer the question whether the adverb structurally in the phrase “structurally modify or upgrade” qualifies only the verb modify or both the verbs modify and upgrade? I have looked at paragraphs 5.143 through 5.161 (15th ed.) but don’t perceive the guidance I need.
A. Alas—the great and powerful manual cannot tell you what this writer was thinking. The only way to know for sure is to ask him or her. If you don’t have access to the writer, then you will have to settle for ambiguity. If you need to know the exact meaning because you’re involved in a lawsuit whose outcome depends on the technical meaning of this phrase, you’re at the mercy of the judge. If you are the judge, well, good luck.
So how do I decide what is the “right” answer? If I am not the author, I ask the author for more information. If I am the author or the editor, I try to write the sentence a different way to see if that changes the meaning or intention of the sentence. I review my source material to be sure I understand what it is I’m supposed to be writing about.
When I was studying French in college, the professor would ask us to answer a question about usage, and then ask why we think our answer was correct. The point was to help us learn how to apply the variety of rules one needs to know to speak (or write) French without sounding like a 2-year-old French child. I don’t recall the exact question now (having to do with liasion), but I gave my answer and then the professor asked, “Why?” I replied that “it just sounds right.” I heard mumblings from fellow students about how stupid my answer was, but the professor said, “Yes! You got it! It sounds right to you, because you’ve absorbed the rule and can apply it instinctively!” (Then he proceeded to ask the mumblers to cite the rule.)
Every day, we apply “rules” of grammar, sentence structure, and style based on our experiences. If we had to “look up” everything, we’d never speak, let alone get any writing published. Those of us who have studied grammar, sentence structure, style, etc. write and edit based on years of writing and editing a variety of works, making mistakes, learning from those mistakes, and remembering the corrections. I can’t cite every rule (I wasn’t an English major), but I know enough about it that if you need a ruling, I can Google the answer for you. (Or better yet, Google it yourself. You’ll remember it better that way!)
We don’t always have a written rule to follow. Styles don’t follow rules (other than the rules set by the person who wrote the style guide.) Do you think e. e. cummings followed rules and styles? Of course he did! But it was a style of his own making, which is not wrong (in poetry) when you apply the style consistently. Usually, we follow the rules for the type of writing that we are writing or editing. That is, the rules for writing technical user guides are not the same as the rules for writing poems or advertising copy (“Got milk?”).
And sometimes we have to go with our gut—or with whatever looks or sounds right.