Say I have a recipe that calls for 5 lbs of LME. How much DME would that be?
In short: 4.2 lbs DME
DME = 0.84 * LME
LME = 1.19 * DME
It depends on the manufacturer. Extracts and malts and adjuncts all have a "points per pound per gallon" rating that you can look up from the supplier. In general, dry malt extract gives you 44 pppg and liquid 37 pppg.
5 pounds of LME gives you 185 points. To get the equivalent points from DME you divide by 44 to get 4.2 pounds.
The conversion rate is 1 DME = 1.25 LME. To get the opposite, you would divide.
For your recipe specifically, use 4 lbs DME.