Ice does two jobs at once: temperature and dilution. The right amount is more than feels reasonable — fill the glass. The paradox: a full glass of ice melts SLOWER than two lonely cubes, so the drink ends up colder and less watery.
Freeze ahead in big moulds: one large cube keeps an Old Fashioned alive for half an hour. Crushed ice for Mojitos comes from cubes in a towel and a few whacks of a rolling pin. And chill your glassware — a warm coupe kills a Daiquiri faster than any measuring mistake.
Homework: freeze a full tray of big cubes and build the same drink twice — two cubes versus a full glass. The difference is the whole lesson.