Half Kelly is the default for practitioners who take kelly-criterion maths seriously and still want to sleep. Growth rate falls to roughly three-quarters of maximum while the volatility of the equity curve halves.
Example: a system with a 55% win-rate and 1:1 payoff gives a Kelly fraction of 2 x 0.55 - 1 = 10% of equity per trade. Half Kelly is 5%. Even that is far above what most discretionary traders risk, which tells you something useful: either their real edge is much smaller than they believe, or their positions are far more correlation-linked than the single-bet formula assumes.
Because Kelly compounds, the practical rule is to compute it, halve it, then sanity-check it against max-open-risk. If half Kelly says 5% per trade and your portfolio rule says 6% total, the portfolio rule wins.
Related: fractional-kelly, kelly-criterion, win-rate, max-open-risk