You should have topped it off immediately when you discovered you overshot your volume. It happens all the time. When you calculate your gravity of your current volume and you then use the dillution tool in beersmith to calculate the gravity after toppiing it off with the amount of water you are missing, you almost always come back to what the original gravity should be. I've topped off when necessary and the beer indeed has turned out fine.
Your beer is exactly stronger by the amount of water you are missing, same if you have to high a volume of water, your beer is exactly weekend by the amount of water you over shot. You can fix that by boiling further to concentrate the wort.
Now, I'd leave it, and drink the stronger beer, and chalk it up to next time.
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