What Drew described is basically what I do. I pull about 1 gallon of first runnings, then boil that down to at least 1/2 the original volume, smaller volume if I can.
I don't care for crystal-type malts, so I almost never keep them on hand. When I do want something with some malty/caramel character, I'll boil some first runnings down. My ordinary bitter is just Golden Promise + boiled first runnings, and I'll boil it down from 1 gallon to 1/2 gallon.
For an ESB I'll pull more volume, say 1.5 gallons and try to get it down to 1/2 gallon or less. I can't describe it more than that, but basically just watch it, keep stirring, and it wouldn't hurt to pull samples, let them cool, then taste them, if you're not sure what you're doing. Let your tongue guide you.