I always find people making up their own interpretation of the 1 inch rule, which is not necessarily wrong but it's just one's opinion. The sticky formula isn't bad, there are a lot of fish keepers with OCTSD, obsessive compulsive tank size disorder. While there's nothing wrong with this, it really all boils down to the fish and it's always relative to the fish. Yes a 10 inch oscar can fit in a 10 gallon because a 10 gallon tank is 20 inches wide. Will it fit ? Absolutely, will it have room to move ? A little but I bet it would prefer a little extra room, know what I mean ? Some believe a fish has these ideas about hey I'd love to have a bigger tank, fact is your fish don't care one way or another. As long as they can swim and eat they will be happy. I prefer to treat fish as fish not as pets. Fish will adapt to anything you throw at them. Tank size is important for the fish but it seems like it's more important to the owner.
My only issue with it is just because some fish zealots feel compelled to run out and buy a 2000 gallon tank for their 1 inch oscar they feel the need to tell everyone else to do the same. Most fish will take a couple of years before they get super large this too is relative to the fish. This gives them plenty of time to upgrade when the time comes, even 1 year is plenty. It's not a big deal. I don't agree with some who buy a fish that will grow big and when it does outgrow the tank they want to get rid of it. I think if you own a fish then you own it for ever so you will upgrade to a bigger tank. A few hundred dollars is not a lot to upgrade to a larger tank and you can find larger tanks on Craigslist for dirt cheap.
I think there should be less judging and more inspiring beginner fish keepers to want to go the extra mile to maintain exceptional well maintained tanks. Nobody likes it forced down their throat.