I think N4HHE has it a bit more right. Regular gas is typically E10 anyways. So when you blend to E30 you are really only adding 20% more ethanol, which is 66% as energy dense. So you are only losing 33% of the 20% vs regular E10 gas, which works out to about 7%.Is it this simple? Just asking because I honestly do not know the answer.
If you were going E0 to E30 then it would be more like 10%