Typical simple syrups are 1:1 sugar to water. I like a lighter hand here, especially since watermelon, in season, is ridiculously sweet. Since you still need enough liquid volume to steep the peppers, I've just reduced the sugar by half (rather than reducing the quantity of syrup, itself, in the agua fresca). And a note on peppers: I tend to err on the side of hot (2 peppers) and just use the resulting syrup judiciously. Always taste your syrup for heat before adding it to your agua fresca. This is also great for spicing up cocktails!