The extract potential of DME is about 1.045. This is important because you'll use the gravity units of that (45 - just subtract 1 and * 1000).
For example sake I'll assume you're aiming for a 5 gallon batch and you're run-off after mashing (before boiling) is 6.00 gallons. To hit your gravity you need to apply the following formula :
((GU of Desire OG * Desired Batch Size) - (GU of wort))/ GU of DME
So, for this example you get :
(68 * 5) - (50 * 6)) / 45 = .89lbs of DME
Thanks to John Palmer's "How to Brew."
I've used this and can typically hit my OG within 1-3 units.
Hope this helps.