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As you start working on your document, there are some best practices that will make life easier down the road, both for you and for anyone who will inherit your file.
Choose the unit you want to measure the document by. I like to work in picas but younger designers work in inches (in the US) and centimeters (in Canada and Europe).
If I’m the only one who works on my document, I can choose picas with no problems. But if I know that several others in my office will be working on the document, it is only polite for me to use the unit of measurement that they are comfortable using. So I ask and we all agree on inches or picas or points and we set them as the preferences for all documents.