the short answer
Pay an assistant coach from the margin their clients generate, not a fixed rule. The four models: a flat fee per client (protects your margin, predictable), a revenue share of each client fee - commonly 30 to 50% for a delivering coach (aligns incentives, but you share every future price rise), hourly or retainer (for part-time or early arrangements), and salary (for a full-time senior coach once volume is stable). Start with revenue share or flat per client, size it so both of you come out ahead, and confirm contractor-versus-employee with a professional in your country.