LME and Specific Gravity and a math question

Recently, I've gone back to brewing with LME and DME as a break from the time involvement of all-grain brewing. I've purchase bags of LME from Williams brewing. They give an average OG of 1.040 for a 6 pound bag into 5 gal. of water, which seems pretty average based on other suppliers' claims. I know there are differences of ferment-ability based upon the grain combinations in the bag, but as a rule, is estimating the OG just a math exercise? For example, does an 8 pound bag give an OG of approximately 1.053.3? I divided 1.040 by 6 pounds, giving me 6.67 of OG for each pound of the LME. I then added twice that back to the 1.040 to give the 1.053.

Am I missing something?