Getting it right - the variables
As with everything in life, practice makes perfect. However, the beauty of this method is once you've calibrated a couple of things, it should work reliably for you every time. The trick is, be consistent and you'll rarely go wrong.
I suggest, the first time you try it, use everything you normally do (unless you don't buy free-range of course!). Keep your usual toaster setting, use your normally preferred bread and use your eggs at the temperature you normally store them at (i.e. either the fridge or at room temperature). That way, if it works first time, you haven't altered anything. There are only two likely things that go wrong, it will be under or over done. Both are easy to remedy. Read on.
So, what are the variables and how do I adjust accordingly?
Size and temperature of the egg | Thickness of the bread | How you prefer your toast | Gas or electric hob | Number of eggs to simultaneously cook
Size and temperature of the egg
Pretty obvious really, the smaller the egg, the quicker it will cook. Similarly, the warmer the egg, the quicker it will cook. So if you keep your eggs in the fridge, you may need to allow a little longer. If you keep them at room temperature decrease the cooking time.
Increasing the cooking time
If you need to cook it longer, put the egg into the water, wait for the water to come back up to temperature before putting the toast on - this will likely increase the cooking time by about 20 - 30 secs. If it is still not enough, next time prepare your coffee/tea in the cup before putting the toast on. Again, this will add another 20 or 30 seconds. The idea is to shuffle the order of things to increase the cooking time.
Decreasing the cooking time
The same applies if you need to reduce the cooking time. put the toast on first, then prepare your coffee or tea before putting the egg in. You can fine tune it by adding or taking off time here and there such as by putting the milk back in the fridge before adding the egg therefore shaving off a little more cooking time.
The thickness of the bread
A thinly sliced piece of bread will take a little longer to cook as it is further away from the elements than a thick piece. You will generally know your own toaster and already have it on the right setting for whatever bread you use. When we use different bread, we tend to know roughly what adjustments to make.
So, if I use a thicker slice than normal, I tend to reduce the timer - therefore, I know that I need to put my egg in the water a little earlier to compensate. Conversely, if the toast takes longer, I'll put the egg in after I've started the toast.
How you prefer your toast
As I've just mentioned, stick to what you like and adjust the cooking time of the egg not the toast. If you like cold toast (French toast) I'm afraid this method is not for you and you'll need to time your egg using a timer or watch.
Gas or electric hob
Gas is the preferred heat source as the alterations you make take instant effect. If you have an electric hob I guess you need to half remove the pan to lower the temperature quickly while the element cools down.
Number of eggs to cook
It depends on how fussy you/your family/friends/guests are about their toast. If they want it hot buttered then you are limited to how big your toaster is. If it's a four-slice you can cook 2 eggs at once. If you want to use the grill as well then you can cook more at once. I personally don't cook more than 3 at once because once you factor in the time it takes to butter the toast and take the eggs out of the pan, it all gets a bit too frantic.
Use a large frying pan and follow the same method but put the eggs as far away from each other as you can.