What is the Kelly criterion telling me?

The share of your capital that maximises long-run growth, given your win rate and your average win against your average loss. Half of it is the number to actually use.

Kelly is the mathematically optimal bet size for compounding — and taken literally it is far too aggressive for real trading, because it assumes your win rate and payoff are known exactly rather than estimated from a few dozen trades.

The card shows both Kelly % and ½-Kelly for that reason. Half-Kelly gives up a little growth for a large reduction in the depth of the swings, which is why it is the one shown in green.

The formula is shown on the card: win rate minus loss rate divided by your payoff ratio. Both figures are computed from your own numbers, so both inherit their reliability — on a small sample your win rate is a guess, so Kelly built on it is a guess too. Treat it as a ceiling to stay well under rather than a target to hit — and note that when profit is concentrated in a few big days, the app's own guidance is not to use Kelly sizing at all, because the "typical" trade the formula assumes does not exist in your distribution.