How it works: Brix = (total dissolved solids ÷ total batch weight) × 100.
Each ingredient's dissolved solids = grams × (Brix% ÷ 100).
Brix% represents the dissolved solid fraction of that ingredient - e.g. glucose syrup is ~80% solids by weight.
Water and proteins (gelatin) contribute 0. Acids and pure sugars contribute ~100.
Refractometer readings on a hot batch will read slightly lower than final cooled Brix.