Well, I think the solution is for enough people to bitch directly to Sherwin-Williams, claiming that the precise one gallon size is important because it allows people to determine surface coverage in a predictable way. Of course, this isn't quite true, because humidity and a few other random things can affect whether seven nineteenths of an ounce makes a difference, but if enough people yell about it, that won't matter.
About 5 years ago, there was a plot afoot to reduce the size of cans of evaporated milk. I heard about it 2 months ahead of time because I'm in the grocery business. It never happened. Initially, I wondered if an army of home bakers got all over the manufacturers' cases, but it might've been bigger than that. Unrelated manufacturers often specify "one such-and-such ounce can of evap milk" in recipes. So, it might've been insider influence that put a stop to it.
The idea made no sense. The vast majority of evaporated milk is sold around holidays. Tell your average grandma that the price of the stuff's gone up forty cents since last year and she'll say "So? It's Easter".