How much is a dash?

I'm really confused about how much a "dash" is.I was making Manhattans using Imbibe's #2 recipe and it called for 2 dashes of bitters and gave a conversion of 1 tsp.I did this, and it tasted great. But I was looking at other recipes, and I saw one that required 4 dashes of bitters. 2 tsp seems like a lot of bitters.Any thoughts or rules of thumb about dash size? http://www.reddit.com/r/cocktails/comments/3eegvz/how_much_is_a_dash/