Well you can always add DME if you are a little short. By a 2-3 lb bag and throw it in the fridge, it will come in handy.
Regarding taking a gravity reading, one of the greatest inventions is a refractometer. It allows you take a reading with only a drop of wort, and I take multiple readings through the whole boil.
If you are using a hydrometer, yes you need to let it cool. I would figure out how much liquid your hydrometer tube holds and then fill it half with wort, and for the rest use (chilled) distilled water. Whatever reading you get you will need to double, but it will allow you to take a reading much sooner.
Typically I brew to a certain gravity (by taking a preboil reading), not a volume. I always hit my SG, but the volume is a little higher or lower.