Most things that I read where the quantity of water per unit of production is given, completely fail at explaining how they got the figure. It is presented in a manner that might lead the completely ignorant to conclude that this water has been completely removed from the hydrological cycle. If it's rain per egg, then this is just another example of someone who has a calculator but no idea what
should be calculated.
A much better measure would be the amount of fossil fuel per egg.
If someone were to grow a turnip, I could go to the trouble of finding out how much sunlight fell on that patch of earth during the growth of that turnip. Then I could compare the total caloric value of the turnip to the total caloric value of all of that sunlight. A very small percentage would result, and the only thing it would prove is that I'm pretty handy with a calculator.