I bake a lot of sheet cakes – 9×13, 11×15 and 12×18… Never a saggy middle :) I’ve used the Heating core, the flower nail and nothing at all and they all come out great. If you bake without the nail or the heating core, reduce the temperature by 25 degrees…that works for me. In terms of a more level cake, I find that baking with the heating core works best. Hope that helps!!