Like the idea of just drawing a line on the egg and waiting till it changes to the right colour, but for those who can’t wait for that to be invented, you could always pop one of these in the pan…

“Heat sensitive color changing indicator signals Perfect for cooking multiple eggs at the same time”

Colour Changing Egg Timer Kitchen Gadget Boil Cook Eggs Perfectly

Link: https://www.amazon.co.uk/Colour-

Colour Changing Egg Timer Kitchen Gadget Boil Cook Eggs Perfectly: Colour Changing Egg Timer Kitchen Gadget Boil Cook Eggs Perfectly: Amazon.co.uk: Kitchen & Home