Co-author splits: 50/50, 60/40, 70/30 and when each one works
The 50/50 co-author split is the lazy default and rarely correct. Four variables decide the right one: who brings the audience, who brings the substance, who does the writing, who runs the launch. Here is the math behind 50/50, 60/40, and 70/30 splits, and the specific situations where each one is actually the fair and smart structure.