Lets say you were aiming for 1.047 but only hit 1.032. In a 5 gallon batch, how much malt extract would you need to add to the fermenter to bring it up to the desired gravity?
If we steal the numbers from this question, we can assume that liquid malt extract has a value of about 37 points per pound per gallon, and dry has about 44. This is not exact.
When we talk about total gravity, we don't need to worry about the effect in volume. The amount of sugar in 50 gallons of water is the same as if you boil it down to 1 gallon. It's total gravity, or total sugar content, essentially.
So, you need to add 50 points to the batch.
I think my math is right. But it's early :) The numbers will honestly depend on the manufacturer of your malt, but you can plug in those numbers instead.
Let me know if that all makes sense.
In my experience:
1 lb of Dry extract will add 44 points per gallon (potential = 1.044), or 8.8 points in 5 gallons.
47-32 = 15 points needed to get where you want to be.
15/8.8 ~ 1.7 lb of DME.
Make a mini-wort - boil and cool the DME in as little water as possible and then add it to the fermenter slowly to minimalize the splash factor.
Y'know, I doubt it really matters if it's perfect down to the third decimal point. Why not just boil 2 lbs of DME in a liter or so of water, throw it in and see what happens? Isn't that close enough? Does it -really- need to be plus or minus .001 of an exact gravity?