I have a copy of the last (4th) edition of "How to Brew" by John Palmer.
According to the book, DME yields 42 PPG (Points per Pound per Gallon), while LME yields 36 PPG. This is an estimation since the exact numbers depend on the extract. As a consequence:
LME = (42/36) * DME = 1.1667 * DME
This is not far from what is stated in the most voted answer here on this topic. The starting PPG estimation there is different so it all makes sense.
Later in the book, in his Porter recipe, Palmer says to either use 6 lbs of DME or 7.6 lbs of LME, but 1.1667 * 6 = 7.
Though it isn't a big difference, I'm trying to understand how these things work: is there something more that must be taken into account, am I missing something or it's just a typo?