Proper measurements are key to creating a perfect cocktail.
The best cocktails are accurately measured and mixed and that means owning a quality Jigger/measuring shot glass.
You needs one at least 1.5 ounce capacity, that's a full jigger. But it must also have a marking for 1 ounce, a.k.a. a pony. Sadly, the numbers wash off many of the glass measures so the tried and true metal two sided jigger from reliable Oxo with measurement etched into the stainless steel and a non-slip grip is the best choice.
This heavy duty Oxo jigger is the same kind the pros use. One side measures a 1.5 ounces with 3/4 ounce and 1/3 ounce increments. The other side measures 1/4 ounce, 1/2 ounce and 1 ounce increments. Dishwasher safe.