What exactly counts as a “dash”?

I’m trying and keeping a registry of drinks that I make, many recipes are from my granddad books which are from 1970-1990. And many of these describe the recipes with a “dash of –alcohol name here–“.

I often use bitters as I do love old fashioned, so I know what a bitter dash is.

But take this one as described in the book:

A1: Shake with ice, 2oz gin, 1oz grand marnier, 1 dash of grenadine, 1 dash if lemon juice. Strain into a cocktail glass and serve with an orange peel.

How much is 1 dash of lemon juice?! Should I use bitters measures here…?!!?