For me that's a couple of basic rules. Rule one: a shot equals about an ounce and a half (two if you're being generous). Ice, mixer and a highball glass, and you have a highball.
Rule two: a basic sour is an ounce and a half of booze, 3/4 of an ounce of a sweet liqueur(s), and 3/4 of an ounce of a tart citrus juice(s) of your choosing. Do it with tequila, cointreau and lime and you've got a (very simple) Margarita. Do it with vodka, cointreau, lime and a splash of cranberry juice and it's a Cosmo. Use lime and rum and you've got a Daiquiri. And an Aviation is a gin sour, using lemon juice, maraschino and creme de violette. Shake 'em up, and strain 'em into a chilled cocktail glass and there you go; the same formula more or less holds for them all. Goose the citrus up a bit and goose the sweet down a tad to your taste. It makes a smallish cocktail, but I'm not of the school that a drink needs to be served in a goldfish bowl. (A good cocktail is a strong drink. Too much of a good thing is not a good thing.)
There are plenty of other drinks that like irregular verbs, make the language of cocktail-age richer and more maddening. Those you will need your books for. But some other drinks are easy to remember because they call for equal parts of all their ingredients: a classic Negroni is equal parts of gin, red vermouth, and Campari--although I've had the drink mixed in somewhat different proportions and enjoyed it.
Here's a drink I found this week that fits into equal parts of all category:
The Cafe Royal Special
Equal parts: Gin, fresh lemon juice, white vermouth, and sloe gin. Shake hard, serve in a cocktail glass, up.
It's an interesting drink, brilliant red as sloe gin drinks usually are, and not stupid-strong. The trick to having it not be too cotton-candy-ish (and sloe gin drinks can be like that) is to use good sloe gin. I have some OK stuff that I got at the local booze shop, but what really works best in this or any sloe gin drink is Plymouth Sloe Gin. It's considerably more expensive than the garden-variety stuff, and absolutely worth it.
See you at Randoradio.com at 4 Eastern today!!