okay I realized the current stat system is too limiting, I’ll try to greatly improve it next update sorry! I see you’re trying to add to the milk stat, but right now you can only return the value.
To get this setup to work, you are almost there, but the milk value need to take into account the previous milkVolume. For example
// Calculate current milk volume
let milkVolume = stats.find(s => s.name === 'Milk Volume')?.value || 0;
milkVolume += (productionRate - expressionRate);