One of the problems with any formula is that it's assumed the water is already clean. I don't care if it's a dew drops of Clorox or a pinch of Hypo it's all guesswork.
My questions to those doing this is what's the bacteria and virus count in the water you're treating? If you don't know how can you know if your putting in enough to kill everything.
And those touting Clorox would likely be ill if the grid goes down. It's constantly loosing strength. The warmer it is the quicker it happens, but it's always happening.
For safe water you want to test it. Residual chlorine is easy to test. Most hardware store, Lowe, Home Depot or pool supply stores all sell a cheap test kit.
With it you can check and be certain that you're using enough or too much. You want a range from.5 ppm to 3 ppm. More or less then this isn't safe.
I work in making potable water and there's no way I could make safe drinking water without this test. We constantly monitor with inline meters and also are required to test every 2 hours just to make sure the inline meters are working properly. If they fail I'm required to test every 15 minutes.
And it's not like clear looking water is always the same. We're frequently adjusting the amount added, occasionally huge changes, though the water 'looks' the same. Muddy water just compounds the problem.
For SHTF situations you'll want to make a solution. Doesn't really make much difference in it's strength but say 1/8 to 1/2 teaspoon to 1 gallon of water. Mix it up and let it stand for a while, maybe 1/2 hour. Most will dissolve but some will sink to the bottom. From this you'll gradually add this to the water you want to treat. Start small so you don't over dose it, like maybe a teaspoon (maybe just a few drops, depending on the strength of the solution and how much stuff is in the water.) Stir it up, wait a few minutes and test. If it's in range you're good to go. If low add some more, if high dilute with more water.
Sorry but there isn't an easier way if you want safe water. Anyone giving you a simple equation to treat water is a danger to themselves and other. They'd never make it in my profession.