If you are just trying to adjust the gravity downward by adding water, I believe it should be pretty simple (someone tell me if I am off base). The problem is knowing exactly how much wort you have in the carboy (or bucket) to make an accurate adjustment. Let's say you have 5 gallons of 1.050 wort, and added 1/2 gallon of water SG=1.000), you equation would be:
((5 gal. X 1.050) + (0.5 X 1.000)) / 5.5 gal. = 1.045
Take your pre-diluted volume times the SG add the volume of water (at SG=1) and divide by your new volume. But if you don't know exactly how much wort you have, your numbers will be off.
If you want to add to the SG, then what double e5 said applies. Though I would note that if you are working with extract, you are generally going to come very close to your SG number, and the reason that you get a high (or low) number is more likely to be the result of not mixing thouroughly enough after topping off (which can be hard to do).
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