Writing percents in novels can be done in a couple of ways. You might simply go with the percentage symbol and the number, like '20%', when it's straightforward. Or, for a more elaborate description, you could say 'twenty percent' and explain its significance in the story. Just make sure it doesn't confuse the readers and enhances the storytelling.
You can write percents like '50%' or 'fifty percent'. Just make sure it's clear and fits the context.