Decide what role prunes play in your recipe (moisture + sweetness, chewy bits, or a jammy binding purée).
Choose the closest substitute from the list and use the 1:1 ratio; if the substitute is tougher/drier, chop smaller.
If swapping with raisins/dried fruit, soak 10 minutes in hot water (then drain) before chopping or puréeing.
For cakes/breads where prunes blend in, purée the substitute until thick and spreadable.
Taste the batter once mixed: if it reads flat, add a pinch of salt; if it reads sharp, blend in a bit of extra fruit sweetness (like a small pinch of brown sugar).