I always just double the recipe for a 1/2 sheet because doubling is easier than one and a halfing. To solve the over flow problem, I just tear off a long sheet of foil, fild it into a strip about 2 inches wide and make a pan extender. It stays put because I grease the sides first. Then I lay my parchment on the bottom. Try it. Been working a charm for me for a whole lot of years.

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