Should I consider water temperature in kettle or in cup to infuse my tea?
75&deg;C is the temperature you want your tea to brew, so if your temperature drops by 10 degrees when you pour it in the cup you'll want to heat it to 85&deg;C, pour it into the cup and let it warm up the cup before you add the tea, that way it is at the ideal temperature.

The other option is heating the cup up with hot water while you heat the kettle, then pouring that water out of the cup just before you add your 75&deg;C water.

I like the first method if you have the measurements down as you'll get a consistent result with fewer steps, but if you're using different cups the second method is more consistent.