If you've ever brewed a batch and come up short on your OG by more than a few points, you may have wanted to fix it by adding some DME. This is a good way to make up for extract that didn't come out of the mash for whatever reason. But before you start weighing out the contents of that spare bag of DME, how will you find out how much you need?
This is really easy. Daniels tells us that we can reasonably expect 45 points per gallon from DME. That means that if we dissolved 1 pound of DME in one gallon of water, we'd have a resulting OG of 1.045. So to make a five galllon batch with this same OG, we'd need five pounds.
Do this first. Divide 45 by the size of your batch in gallons. I typically do five gallon batches, so 45/5 = 9. This number just tells me how many points I can expect to yield when adding 1 pound of DME to a five galllon batch. Makes sense too - add five pounds to five gallons at 9 points per gallon, and it's easy to see why your OG is 1.045.
So if you happen to come up short 9 points, just add one pound. But if you need more or less, what then?
Easy. Divide the number of points you need by the number of points yielded from one pound. That is, if you need 5 points, divide 5/9 = ~0.56 pounds. If you need 11 points, 11/9 = ~1.22 pounds.
Easy as 3.14159265....
If you'd prefer not to do the calculations, and you also happen to brew 5 gallons batches, I put together a table that shows the amount of DME you'd need to add in order to get a certain yield.
 Daniels, Ray Designing Great Beers (Brewers Publications, 1996) p.31 - Table 5.1