I'm just estimating, but 1 pound DME in 1 Gallon gives about 1.040.
So, 5 pounds in 5 gallons gives 1.040.
So, about 8 points per pound. (in a 5 gallon batch)
You have about equivalent of 3-1/2 pounds in your wort.
I'd add 2 pounds and call it good.
NOTE: I frequently use Tastybrew.com
Try the gravity calculator, enter 5 gallons with 3 lbs of extract, then nudge it up a bit.