You didn't do anything wrong. It's to be expected that an extract batch comes up a little short. You just top it off with water. You can even you the topping off with water as part of the cooling off process by tossing in ice (prepared in a sanitary manner) or by putting refridgerated ice water in the fermentor (assuming you don't put in more than your boil-off).
But since you didn't top off your beer will be a little stronger and fuller bodied and a higher o.g. I'm not sure about final gravity but it will probably not be the targeted gravity. After two weeks when you think it is done take gravity readings two or three days apart. If the readings stabilize (are the same) then it is ready and that's your actual final gravity.
To figure out your original gravity: Well, let's suppose your instructions said it was supposed to be 1 gallon at O.G. of 1.042. Subtract the 1 and multiply by 1000 to get 42. That's the "gravity points". Volume x Gravity Points will be the same for a batch of wort no matter how much or how little water you boil away. So take your *actual* volume (if you measured it). Let's suppose it is 3 quarts or 3/4 gallons.
This means 1 gallon x 42 gravity points = 3/4 gallon x YOUR GRAVITY POINTS. This means YOUR GRAVITY POINTS is 56 (because 1 x 42 = 3/4 x 56). Divide by 1000 and add 1 and your O.G was actually 1.056.
But if you didn't measure your batch ... well, forget about it. You just won't know what you o.g was (except that it was higher than the instructions said).
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