I don't use LME anymore, but see below for DME ratings.
As a point of reference, American 2-row pale malt in its pure form is approx. 34-37 ppg and about 2 L.
Briess Pilsen Light DME = 44 ppg / 2 L = (American Pils Malt & Carapils)
Briess Golden Light DME = 43 ppg / 5 L (American 2-row, Carapils, & Light Crystal Malt)
Muntons Light DME = 44 ppg / 5 L (English 2-row, Carapils, & Light Crystal Malt)
Muntons Extra Light DME = 37 ppg / 3 L (English 2-row & Carapils)
The higher the L, the darker the extract. Adding half (or more) of your extract very late in the boil helps to minimize wort darkening, among other benefits. The ppg rating has to do with the amount of available sugars in the extract. For example, if the ppg of extract is 1.037, then putting 5 pounds in 5 gallons of water (1 lb/gal) would yield an original gravity of 1.037.