Most of the time these fractions add up to a whole, but sometimes they
do not.
First off, some cocktails are written in "glasses" or "Wine Glasses".
Usually, but not always these are what I call "Party Cocktails".
Cocktail recipes written for 4-6 people.
According to a post from David Wondrich on eGullet:
"The "wineglass" used to be a standard unit of measure, and it was 2 oz. Towards the end of the nineteenth century, as it fell out of use, some people interpreted it as an actual wine glass, and considered it to be 4 oz.
"At the same time, and I'm sure not coincidentally, the 2-oz metal
jigger was introduced, and soon became the standard bartender's measure.
Around 1900, this got supplemented by the 1 1/2 oz jigger, for
cheapskates or lightweights, and even the 1 1/4 oz jigger, which was
popular, we are told, in the bars around Wall St, as brokers liked their
drinks small, but frequent."
So after making a few of these party cocktails based on a 2 oz "glass", it is pretty easy to start to get a feel for the size of the average Savoy Cocktail.
The Jewel Cocktail, for example, makes the math pretty easy:
Jewel Cocktail
(6 People)
2 Glasses Green Chartreuse.
2 Glasses Italian Vermouth.
2 Glasses Gin.
1/2 Dessertspoonful Orange Bitters.
Shake thoroughly and serve with a cherry, squeezing lemon peel on top.
6 glasses of 2 oz each makes 12 oz of liquor before dilution. That makes an individual cocktail portion 2 oz per person.
In addition, Robert Vermeire, a continental contemporary of Craddock's, wrote his cocktail recipes based on fractions of a half gill total pre-chill volume. A half a gill is a bit more than 2 oz.
So this really makes most cocktail math pretty easy.
Say, for the following:
Jeyplak Cocktail
1 Dash Absinthe.
2/3 Dry Gin. (1 1/2 oz Gin)
1/3 Italian Vermouth. (3/4 oz Vermouth)
Shake well and strain into cocktail glass. Squeeze lemon peel on top.
That's a total volume of 2 1/4 oz before dilution, pretty close.
The following volumes then work for most cocktails in the Savoy:
1 glass or Wineglass is 2 oz
2/3 is 1 1/2 oz
1/2 is a generous 1 oz
1/3 is 3/4 oz
1/4 is a generous 1/2 oz
1/8 is a generous 1/4 oz
I have to admit Sixths are a little tricky. 1/2 of 3/4 oz is kind of
hard to figure out in absolute volume or in fractions.
But it isn't hard to get close using a 3/4 oz jigger.
For dashes, I also defer to Vermeire, who called them at 1/3 of a teaspoon for his 1/2 gill cocktails.