This is not from ALD, historically the pilots would start with an actual landing distance and then factor this with a factor depending on the failure. Now days post roughly 2012, the in flight landing distance is already made more realistic (more factors built into the calculation in accordance with the TALPA/ARC model). This is to make it more realistic of a line pilot. But as far as flight crew are concerned the result of an in flight landing distance is not factored! Therefore according to airline policy an additional factor is added. Airbus recommend 15% but this can be removed in the case of exceptional circumstances.