I was recently reading a book about Trader Vic's Tiki drinks and it defined a "dash" as one and one half teaspoons. Later in the book I found a recipe that called for two dashes of bitters in a single-serving drink.
That's freakin' PREPOSTEROUS! Three teaspoons is one-half ounce. I doubt there are many people who could drink a potion with that much bitters in it.
I define bitters by the drop. Generally, one or two drops of Angostura or Peychaud's bitters is plenty for my palate. Of the two brands, I prefer Peychaud's.
There are about 560 drops (OK, call it 600 like everyone else does) in one ounce. A different on-line bartending reference says that a dash is 1/32 of a teaspoon, which would make it about three drops. I think that bartender had his thinking cap on.