DIY
Temperature, time and ratio. Three variables, and the difference between them is the difference between tea and hot bitter water.
Strength is a ratio, not a duration. One bag in 200 mL is a different drink from one bag in 350 mL no matter how long you leave it — and leaving it longer to fix a weak ratio just extracts bitterness.
Cold milk drops the temperature immediately. Add it early and the tea brews cooler and slower; add it late and the leaves spend the whole steep at full temperature. The tool models both.
Freshly boiled tea is around 95 °C and genuinely scalding. It follows exactly the same cooling curve as a chilling drink, just in the other direction — so the same tool tells you when to pick it up.
While the tea steeps, the egg timer handles the other half of breakfast — and it accounts for altitude, because water boils cooler high up and a "three minute egg" is a sea-level idea.
The same cooling curve that chills a beer tells you when the tea is drinkable, and the same altitude that ruins your kettle ruins your eggs. Change the vessel size and the ratio follows.