Nobody can guarantee to ALWAYS remember to short tie the dinghy before backing up.
That is why a short painter is advisable. Measure the distance between the closest point of attachment of the dinghy end of the painter and the nearest propeller. Make the painter slightly less than double that length.
In my case that distance also works out to allow me to attach the boat end to my midships cleat and have the stern rest against the side of the swimgrid, when at anchor. That makes boarding simple and leaves the swimgrid free. Towing the dinghy at 8 knots, it is vary well behaved on a ~15' painter and I have never had to worry about backing over the painter.