LME is part water and is therefore heavier per sugar content than DME. Truth is, different brands and types of LME may have slightly different water/extract content. The usual rough estimate for converting DME to LME is 20%-25% more LME by weight than DME. A better way to figure out how much you need it to use brewing software and target a specific OG. E.g., BeerSmith tells me that 6lbs of XLDME in 5 gallons gives me an OG of 1.052, while 6.9lbs of Pilsen LME also gives me an OG of 1.052. But fretting over fractions of an ounce is pointless, and LME is often sold in 3.3 lb (1.5 kilo) cans. 2 of those gives me 6.6 lbs and an OG of 1.050. Maybe that's good enough? If I really want to hit 1.052, then I can add 0.25 lbs (4oz) of DME.