I was just promised some sour cherries from Door County. As it happened, I had just made pectin stock from underripe cooking apples and had been thinking of a cherry-blueberry jam. I went on line and found a recipe that called for 225 grams each of cherries and blueberries, one green apple, and the juice of one lemon. After simmering the fruit until it is soft, the recipe calls for measuring the pulp and for each 225 ml of pulp (a volume measure), adding 175 g (weight) of sugar. Why not simply specify 40% of the weight of the raw fruit as the weight of the sugar? You can modify the recipe size easily using weight percentages.