I think you mean something that maximizes the bottom x percentile (either by making the negative smaller or the positive bigger). It's not going to be an easy

I think you mean something that maximizes the bottom x percentile (either by making the negative smaller or the positive bigger). It's not going to be an easy solution, because for a sufficiently small x, the optimal strategy for a small number of bets is going to be to bet $0 each time (since the x-th percentile of the terminal outcome will be a loss on capital, or ruin), but the optimal strategy for a large number of bets is going to be to bet > $0 (since the x-th percentile of the terminal outcome will be positive). So we can already see that this is going to be a function of the number of bets, unlike with Kelly, and therefore is going to be a much harder problem.