This formula can be used with just lemon, lime, or a combination. The ratio of ingredients by weight is - Juice : Brown Sugar : Water : Rum = 1 : 1-1/4 : 4 : 1/3
3 small limes and 3 small lemons are used here. First, zest them all into a pot. The pot size is at least 1.5 L or 1.5 quart. Juice all fruit and discard seeds and big chunks of tissue fell in the juice. There are many ways to juice citrus, from simple to fancy. The tool shown in mid picture is what we have on hand. It loosens pulps into the juice giving the ice a very nice texture. All juice weighs about 200 grams (the container is 86 grams).
According to the 200 gram juice weight, we can calculate the rest of ingredients: 250 grams of brown sugar, 800 grams of water, about 70 grams of rum. In the pot we zested in the citrus, pour in sugar, water, then heat until sugar dissolves completely.
While the liquid is heating up, pour rum into juice. We found dark rum tastes better giving a hint of molasses. When sugar is completely dissolved, turn off heat, let it cool down to room temperature, then pour in the rum-citrus mix. The vitamin C in citrus is better preserved if the juice is not poured in hot liquid. The texture looks nice with zest and pulps in it.
Separate the lemonade into several containers and place in freezer. A shallow layer allows it to freeze faster. About 24 hours later, the lemonade becomes solid but not rock hard thanks to the rum. fluff it with a fork and put it back to freezer. Enjoy whenever you feel like having some refreshment!