it really depends on the type of cookie you are trying to make. Also it can depend on the sugar to fat ratio. Also to an extent the altitude in which you are baking. Depending on the type of cookie, i would start toying with around the 15% mark and go from there. You will see differences with the texture of the batter while mixing it (too loose, runny etc). Also consider the type of liquid you are using. If it's dairy, try almond or rice milk. They have a natural sweetness to them, however using them will change the flavor and since the fat content is different can/will change overall mouth feel.