Use abbreviations and numbers according to the conventions of a specific field (see p. 740): for example, 57% might be acceptable in a math paper, but 57 percent may be more appropriate in a sociology essay. (61g)
If you use an abbreviation readers might not understand, spell out the term the first time you use it, and give the abbreviation in parentheses. (61c)
If you use an abbreviation more than once, use it consistently.
Consider your context. In informal writing, abbreviated terms and symbols are much more conventional than in formal writing. (30a)