One way I use math in writing is to determine pacing and plotting. I don’t want to use up all my planned plot points too quickly…or have them all at the end in a big dump. I know I’m at the end of Act I right now. I know what my midpoint action will be, and where in the word count it will fall. I can figure out how much narrative and how much action comes between now (18,000 words) and then (35,000 words).