A position-sizing formula that returns the fraction of capital to risk per trade given an edge: f^* = W - (1-W)/(b), where W is win probability and b = R_win/R_loss is the reward-to-risk ratio. Pure Kelly maximises long-run growth but is volatile; practitioners use half-Kelly (f^*/2) to cut drawdown roughly in half at the cost of a small growth-rate reduction. Inputs must come from a credible sample of historical results, not optimism.