Water addition to wort calculation

I have an original gravity goal and am extract brewing. My boil volume is 3 gallons. After taking a hydrometer reading from the wort, what is the calculation to determine the exact amount of addition water to add to the fermenter to meet my desired OG?

Is it an extract kit?

If so, then they are designed to be a 5 gallon batch size and that means top off post boil volume with enough water to reach the 5 gallon mark on your fermenter and your OG should be right on.

No. Not a kit. I've done 4 extract batches with recipes so this is my 1st personally written one. I've figured out what I want my gravities to be at each volume, but with the possibilities for changes from boil evaporation, miscalculations, being new at it, etc, I wanted to be able to be as close to my goal as I could because I know too much and ill go under and too little and ill be too high. I have all grain brewed multiple times in the past and used a calculation to figure this out but I can't remember it!!

You take the total gravity points that you have from the extract and divide it by the total volumes:
ie) DME is roughly 46 points per pound PER gallon
So if I have 5# of DME (5x46 = 230 points)
in 1 gallon OG = 1.230
2 gallons 1.115
3 gallons = 1.077
4 gallons = 1.056
5 gal = 1.046
6 gal = 1.038

So after the boil, all you have done is boil off water, you have the same amount of sugar still in the kettle so top off until you reach the desired OG

brewkinger said:

You take the total gravity points that you have from the extract and divide it by the total volumes:
ie) DME is roughly 46 points per pound PER gallon
So if I have 5# of DME (5x46 = 230 points)
in 1 gallon OG = 1.230
2 gallons 1.115
3 gallons = 1.077
4 gallons = 1.056
5 gal = 1.046
6 gal = 1.038

So after the boil, all you have done is boil off water, you have the same amount of sugar still in the kettle so top off until you reach the desired OG

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Thank you!