What you call a "shot" varies in North America. A standard shot is 1.5 ounces. A pony shot is 1 ounce. A double is 3 ounces.
A standard shot measuring unit has a 2 cl and a 4 cl part.
- 2 cl = 2/3 oz = small shot
- 3 cl = 1 oz = 1 pony
- 4 cl = 1.333 oz = big shot
- 6 cl = 2 oz = one small, one big shot
- 6 tsp = 1 oz = 3 cl
- 2 tsp = 1 cl
Hence, when I make a whiskey sour with 2 oz whiskey, 2/3 oz lemon and 2 tsp sugar, this is actually 6cl, 2cl, 1cl or a 6-2-1 ratio.
The classic David Embury sour ratio is 8-2-1, though I believe his sugar syrup is more concentrated than 2-1(but probably less than 3-1). So his 8-2-1 ratio might be a little bit sweeter than you may imagine.
To make the classic Embury sour, use 4 cl of liquor (big shot), 1 cl lemon or lime (half a small shot) and 0.5 cl (1 tsp) of sugar syrup.