Notice that in the current comics, you occasionally have heard "prime Earth", but we no longer hear of "Earth 1", except for that weird series of trade paperbacks that don't all take place on the same Earth.
If the Arrow/Flash universe is the prime Earth, I'd say THAT has to be thought of as Earth 1. Haven't they even used that term? "Earth 1"?
I'm fine with saying there's no other Earth that can be called "1". Just move on from this point. "1" suggests "the beginning", so there can't be any other but the one they've just identified, the Arrow/Flash universe.
I'm okay with Jessie and Harry's Earth being Earth 2, Jay's being Earth 3...
I'm just having a problem calling Kara's "Earth S" since that designation is usually kept for the Shazam characters' world. Maybe "Earth 5"? It looks like an "S", but it's also the number of sides on the traditional "S" symbol.
Think that Kara earth was be retconned into her always being on Flash earth, due to Barry causing a Flashpoint event!