now that all the "Granny Science" is out on the table, we can get to the actual science of why everybody's Granny has a different fool-proof method which does not work for anyone else....
you're in Rome. do you keep the eggs on the counter or in a fridge?
the starting temperature of the egg does influence the timing of whatever method you choose.
(we got a new refrigerator - and I had to adjust the cooking times I'd used for 15 years......)
starting in cold vs. hot / boiling / simmering water:
how cold is cold, how big is the burner, how much cold water are you heating, how many eggs are you cooking?
it makes a difference - longer heating time = longer time for heat to transfer into the egg.
the inconsistent amount of cold water is the biggest factor.
"cover eggs one inch" - works for same pot, same water temp, same number of eggs.
use the same pot with 2x the eggs, less water needed to cover, heats faster, eggs underdone.
turn off and let stand....
as experience goes, you may have noticed a large volume of water cools slower than a small volume of water. (actually depends on the "surface area" of the water volume, but as a general rule for common pot sizes / shapes.....)
eggs will be over / under done as those factors change.
rubbery white, etc.
rubbery whites are due to over cooking
the green yolk ring is due to over cooking
it is possible - by too high temp cooking - to overcook the whites before the yolk is 'done' -
heat only transfers into a egg from the exterior at a given rate - that rate depends on two things: heat transfer coefficient and temperature difference (outside to inside) - aaah, surface area as well - the pewee to jumbo size thing....
problem: the transfer of heat slows down as the white / yolk begins to solidify. a sustained 'overly high' exterior heat input can overcook the white before the heat has time to get to the yolk.
cook a steak / roast too fast = same effect - burnt on the outside, bloody on the inside.....
of course, none of the egg cookers have ever done that, eh?
boil / simmer / steam
water boils at 212'F / 100'C at sea level.
all the water in the pot is _not_ boiling - it's actually somewhat just below boiling temperature.
the water molecule H2O exists at 212'F/100'C in two forms: liquid and gas (water and steam) - it's the "bubbles" thing . . . .
there is a seriously major difference between the two phases. it takes a lot of heat energy to make water at 212/100 change from a liquid (water) to a gas (steam) at the same temp 212/100.
(latent heat of vaporization, anyone?)
so when steaming, the steam condenses on the cold egg and although the temperature of the steam is the same, condensing steam imparts a lot more heat energy to the egg and the egg cooks faster. and is more prone to over-cooking.
oven cooking -
same deal. if the temp is too high, rubbery whites, raw yolks.
green rings... over cooked is over cooked - it's a chemical reaction, sulfur + iron thing - the heat source does not matter.
there's a good explanation with pictures for the Granny challenged here:
the only actual "fool proof" method is to start with boiling/simmering water, start timing from when the water returns to the prior state, experiment to determine the right timing for egg size and egg starting temperature at your altitude.
that you can replicate at any time with any number of eggs in any size pot.
I take a short cut - because my usual is three eggs in the same pot with roughly the same amount of water from the same spot in the same refrigerator. water gets to a mild boil, in go the eggs, set timer for 16 minutes, remove immediately & chill shock.