The Tur starts off by citing the opinion of the Ramban that the prohibition to wash and anoint starts soon as you start the se'udat ha-mafseket since at that time you are implicitly starting the 9 be-av mourning and that the earliest such time is from chatzot ha-yom. But how can you sart a meal at the end of the sixth hour within a half hour of minchah gedolah? Wouldn't the earliest time to start se'udat ha-mafseket be at minchah gedolah, right after you daven minchah?