In his second book, The Curious Cook, McGee kindly does these calculations for us, listing multiple fruits and three consistencies of ice and how much fruit, water, sugar and lemon juice is needed for each. Cranberry sorbet presents a special situation, though, if you want to use the fresh fruit. Pureed uncooked cranberries separate into unpleasant dry bits and liquid—not good for a sweet frozen dessert. Cooked, though, as the cell walls break down, two significant events occur. The fruit becomes soft enough to be pureed into a smooth mixture, and pectin is released to thicken it. But that pectin throws off the straight sorbet calculation, because will keep a less sweet sorbet creamy at a lower temperature. You also need to add back the water that evaporated off during cooking.