Depends on what you want, but I think this is probably nearly it.
I'd do it like this (roughly).
For: "Order", 1 to LayoutHeight / 20
System: Number 'Object.Y' is between LoopIndex("Order") * 20, (LoopIndex("Order") * 20) + 20
Sprite/Family: Set 'Zorder' private variable to LoopIndex("Order")