Yea, based on the eia data it should all be ballpark but the calculation is there mostly just to serve as an example. You could shave a little off gas-per-fillup in retrospect because very few people will use 15gal exactly and few new car gas tanks are over 15gal but the $3 service charge balances that out somewhat. Based on the map on their site, it looks like their margins might be slimmer than what I calculated but they succeed by having a lot of customers in a very small area that they serve.