Adjusting gravity with more H2O

With my last couple of batches I have added only enough water to my primary to about an inch below the 5 gallon mark, then take a gravity reading and add more water if not up to my desirerd OG the recipe calls for. What I need is a chart or "rule of thumb" that shows me how much water will lower my gravity reading by how many points. For example 1/2 gallon of water might lower the gravity by .004 etc. Is there a simple chart for this on-line?

You need to know the points per pound per gallon for the type of sugar you are using. Me, I would invest in BeerSmith or similar brewing software.

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).

Another vote for BeerSmith. Best $20 I've spent on brewing to date.

what duck assassin said is correct; I love beersmith, but there's no need for it for something as simple as diluting wort.

initial volume X initial gravity = final volume X final gravity