Nobody can remember the exact amounts that go into a Harvey Wallbanger. Not all the time anyway. The secret most bartenders hold dear in their hearts is that they know how to make a very small list of drinks by heart and the rest they will either take a stab at or look up. What they consider a small list might put the novice into fits of catalepsy. In return they often have to mix very few things outside of their comfort zone. People are shockingly comfortable with rum & coke, screwdriver and gin & tonic. None of which require even the slightest effort to remember. Doing them well, is another story entirely, and relies on the quality of components as well as having a steady pouring hand.
For those of us who want to keep something in your pocket a little smaller than The Savoy Cocktail Book, I use Mr. Bartender. As far as free apps go this one does about 99% of what you could want. It keeps a Kitchen of what you’ve got in stock. Lets you search by name, ingredient or even combinations of ingredients. And once you’ve gotten your bar loaded up you can just shake the phone and have it generate something random from what you’ve got on hand.
It also provides pictures of what the drink looks like, which can help a lot for layered shots. If there isn’t a picture you can make the drink and submit your own shot.
And should you get a little creative you can submit your own creations including photos.