With every cocktail you ever make, if it includes ice in the final glass, fill the glass completely. This is a mistake that a lot of people make. Filling the glass all the way keeps the drink at the proper temperature, releases the proper amount of water to help the ingredients blend, and allows for the correct drink proportions in some cases. Don’t worry about it watering down your drink — it will melt a lot slower than smaller amounts of ice. You’ll actually wind up with a stronger drink if you use more ice.
The other time you’ll be using ice is with shaken and stirred drinks that you then strain into your serving glass. This process isn’t to mix the drink, but to cool it down and once again to release a small amount of water to help the ingredients blend (this is why you should still stir or shake a drink with ice even if you’ve chilled all of your ingredients beforehand — hat small bit of water is essential). In these cases, fill up your mixing glass or shaker anywhere from half the way full to all the way full with ice. Some recipes might specify how much ice to use at this stage, but generally you’ll be alright if you stay in this neighborhood.