For me; Ideally, it should hang 30cm on each side of the bed.
The standard height of a bed is 55cm. If your bed measures 150cm you have to think that from floor to floor we would have 2.60cm. As we don't want it to drag, but we don't want it to look like we have it from a previous smaller bed, we choose 30cm + 150cm (size of our bed) + 30cm = 2.10cm but as we always have to be very generous, give it 10cm more = 2.20cm because it's better to have too much than too little.
Elena.