Having been a serious observer of teatime all my life (my mother is a proper Englishwoman), I probably pay more deference to the pot warming and steep timing rituals than most. However watching the white-tie sommeliers of Paris’s Mariage Frères brandishing their silver-cased, pocket thermometers inspired me to pay greater attention to correct water temperatures as well.
Does the heat of the water make a difference in the tea’s taste? There’s certainly debate about it, but it makes sense not to scald the subtleties out of delicate leaves with boiling water (green tea will go bitter), while other teas need that kind of rough coaxing to reach their potential.
Homesence has provided an easy to read chart for brewing the perfect cup. It’s reliably accurate although I think you still need to experiment to find what suites your own taste. After all, “black teas” can include fragile, first flush Darjeeling as well as a brisk, broken leaf Ceylon.
Any water thermometer will do the job. Or you can use an electric kettle with temperature settings such as this one by Cuisinart. It’s probably not as accurate but it has a hold/warming button that will keep temperatures steady if you become distracted by other things.
And unless you live at Downton Abbey, that happens all the time, right?