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A tablespoon of vinegar per litre, give or take. (1 Litre = approx 4 cups)
i prefer not to use any.with very fresh eggs no need. with older eggs i agree with the ratio above. i find the vinegar masks the flavor and toughens the egg. Also keept the water at a simmer. no boiling/bubbles.
When I worked a busy breakfast line, they used to put just a splash of vinegar in the pot to lightly layer the bottom of the pot before you add the water and bring it up to a simmer. They key, is a low simmer definatley.
It also helps to crack your egg in to a small bowl first and then pour it in and also make sure to use a slotted spoon to take it out of the water. I've heard that Vinegar helps keep the white held together to the yolk. But i do agree that too much vinegar can make the eggs taste bad, just go really light on it.
Just in case you need a few hints: Use the freshest eggs possible. The whites stay together better. To make my egg whites effectively cling to the center I tip my eggs into a pot that I have seconds before; twirled the water with a large spoon. To ineffect make a vortex. This pulls the egg towards itself and clumps it. Nice thing about poached eggs, is you can make several and then reheat in a pan of warm water. Make sure your pan has at 3" of water. You don't want the egg to cling to the bottom of the pan.