The example above is only a sell.
You need to do a buy too (how much USDT will it take to get 1 ETH).
And then calculate the difference.
As I said, the ETH-USDT major pair will have a relatively small spread. Do the same operation with the smaller pairs and I'm sure you'll find that they have much bigger spreads.
RE: An Example How Uniswap Calculate The Price Of The Tokens and What Happens When A Swap Is Made