When I learned about these facts about water temperature my first though was, "Thank goodness I love black teas-no need to worry about temperature". My next thought was "How the heck do you figure out what the exact water temperature is?"
One way to make certain that you're heating your water to the correct temperature is to use a tea kettle with a digital temperature display. Set the machine for the temperature you want and it will click let you know when that temperature is reached.
However, you don't need a fancy machine-you can use a clear tea kettle and the water itself will tell you when it's ready. Watch the kettle. When little bubbles start to form you're ready for green teas. When the bubbles start bouncing a bit more, it's time for your oolongs. When the water hits that rolling boil the whistle will probably start to chirp announcing that it's time for black tea.
Honestly I use my inexpensive clear kettle more than my digital one. Waiting for the water to come to the proper temperature is a meditative process and it's fun to watch the water dance.