Quick answer: the classic sour formula is 2 parts spirit : 1 part sour : 1 part sweet (e.g. 50 ml gin, 25 ml lemon juice, 25 ml syrup). It's the starting point for most cocktails — adjust from there to taste.
Sour and sweet work as a pair: lemon or lime juice alone is harsh, syrup alone is cloying. Together they create a 'delicious' brightness. If a cocktail feels sharp — add 5 ml of syrup; if cloying — 5 ml of citrus juice.
Bitter is what makes a cocktail 'grown-up': bitter liqueurs, vermouth, tonic. Bitterness stretches the flavour and removes the lemonade feel. Try the same highball with soda versus tonic — the difference is exactly the bitterness.
Strength is not just ABV but also the body of the drink. Dilution is critical: 20-25% of a finished cocktail's volume is meltwater from ice, and it's part of the recipe. That's why a cocktail without enough ice tastes 'heavy'.
Palate training: make one sour with the 2:1:1 formula, then change one variable (more sour, less sweet) and compare. A few iterations teach balance better than any theory.
BrandBar syrups keep consistent sweetness from batch to batch — recipe proportions reproduce exactly, without the surprises of homemade syrups.