I don't think I buy that 13.56 ml of 1:1 results in the same sweetness as 10 ml of 2:1, unless I'm totally missing something. The reason is that no matter the brix sweetness of the syrups undiluted, the sweetness after dilution into the cocktail will depend on the number of sugar molecules per ounce in the cocktail. Using twice the volume of 1:1 gives an equal number of molecules per ounce in the cocktail as half that volume of 2:1, so it should have exactly the same sweetness in the cocktail.
But I think maybe I have to vote down my own comment! I didn't take into account that 2:1 doesn't automatically mean the concentration is twice as great! I bet 2 cups sugar + 1 of water gives greater volume than 1 cup sugar + 1 of water, so you have to add less 1:1 to get the same sweetness as you would think. If "brix" is actually a direct measure of sugar concentration, then the 13.57 ml equivalency would make sense!
