A longtime, highly skilled editor I know likes to keep track of how much she's tightened a manuscript, and she does it by counting words or pages as she works. This is especially useful if a publication (a magazine, for example) has only so much room for a particular article. But it may also be useful in editing long documents, such as books. After you've done it for a while, you'll get a feel for how much certain kinds of material need to be tightened, and you can use that as a guide in the amount of editing you do.
If you think this technique would be useful, Microsoft Word makes it easy to try. Before you start editing a particular document, do this:
1. Click "Tools."
2. Click "Word Count."
A dialog box will appear that shows the length of your manuscript in pages, words, characters, paragraphs, and lines. On a sheet of paper, jot down the number of pages (or words, if your document is short). Then, as you edit, check from time to time to see how you're progressing. Unless your space is limited, you don't consciously need to cut by a certain number of words or pages. Just edit as you ordinarily would. When you're finished, check the page count again. How did you do?
You might want to keep a record of your results for a variety of documents. Eventually, it will help you know ahead of time if you can get a chapter or article down to size through your regular editing, or if you'll need to get out the ax and start chopping. If you're negotiating with an author or client, that may be a useful thing to know. It may also be useful if you're making assignments to other editors: "Will you cut this by a third, please? I think that would be just about right."