How much ganache (white chocolate) will I need to cover a rectangular cake that is 12x9, 3 layers (6") high? (I baked two 1/2 sheet cakes, cut in halves and stacked 3 halves to make the cake.)

The ingredients are expensive enough that I don't want to make extra, but want to have enough so that it can sit overnight before I use the ganache. Thank you for your help!!