Lowering OG with H2O

Is there a rule of thumb to follow when lowering OG of a 5 gallon batch once it is poured into the primary fermenter? A few times I have had a OG higher than the planned OG in the recipe and added H2O to lower it, but I am only guessing how much to add. It would be much easier to have a measured amount based on needed drop in gravity. How much would the gravity drop per cup, quart, or gallon of H2O?

There are several calculators on line that help with volume change and OG.

http://www.brewheads.com/newvolume.php

You would need to have a fairly accurate way to measure the volume of wort your trying to dilute. After that it's simple math.

say you want to lower a 1.050 to become 1.040. this is a 20% reduction. so add water to the tune of 20% of the current volume of wort. If you already have 5 gallons of the 1.050 wort then add 1 more gallon to get 6 gallons of 1.040 wort. I think that's how you'd do it. But the wild card is guessing the amount of wort you have before you can calculate how much water to use to dilute it.

Are you doing partial mash extract brews? If so, you're probably hitting your OG and just getting a skewed sample when you check your gravity.

[initial OG]*[initial volume]=[final OG]*[final volume]

so [water to add]=[initial OG]*[initial volume]/[final OG] - [initial volume]

Note that all OGs are in points, so 1.050 = 50 in this case.

So you wanna go from 1.056 to 1.048, and you have 5 gallons in your fermenter:

water to add = 56*5/48 - 5 = 1.22 gallons