The Chicago Manual of Style is what Inkd uses for fiction. I am amazed at how many editors fail to fully understand the basics such as when to use a numeral instead of spelling the number in a word. Some aspects, such as percentages, are more complicated, but the updated version should be a professional editor has access to.

Writers, however, rely on editors. Here is one that is often wrong.

Normally, small numbers zero through a hundred are spelled out in nontechnical (fiction). There are exceptions.

In section 9.20 of the CMOS, it is stated:

Except at the beginning of a sentence, percentages are usually expressed in numerals.

So, after being trained not to use numerals, with percentages we do. However, we don’t use the symbol, but spell percent out.

In nontechnical contexts, the word percent is generally used; in scientific and statistical copy, the symbol % is more common.

Here are examples of nontechnical (fiction) usages

• With 90-95 percent of the book in first draft, I can see the end.
• A 70 percent likelihood of rain is forecast for today.
• His new car carries an interest rate of 4.3 percent.

(beginning of sentence exclusion)

• Twenty-four percent of the voters made a bad decison.