Combine 600 g granulated sugar and 250 ml water in a heavy-based saucepan over low heat, stirring continuously until the sugar fully dissolves.
Increase the heat and bring the liquid to a rolling boil without stirring.
Mix 5.1 g milk with 5 ml (1 tsp) of water and pour it into the boiling syrup, then simmer for about 10 minutes as impurities gather at the surface.
Pour the syrup through a piece of muslin cloth to filter out the scum, taking care not to press or squeeze the fabric.
Wash the saucepan, return the strained liquid to the pan, and bring it back to a boil.
Stir in 5.1 g lime juice, then skim off any remaining froth with a slotted spoon.
Check the consistency by carefully dabbing a cooled drop of syrup between your thumb and index finger and parting them to observe the number of threads formed.