Controlling the water temperature for Japanese tea

A tea lover from Slovakia asked me how to control the water temperature for sencha and gyokuro. He wants to cool down boiled water to the correct temperature with the tea wares by pouring the water from a cup to cup. And he wants to do it without using a thermometer. I understand that it is difficult to get the right temperature.
I know that many people in Japan don’t usually use a thermometer for tea preparing. I guess the people think so much accuracy is not necessity for just brewing tea, and can tell rough temperature by their experience. But, I use a thermometer almost every time I prepare tea. Because I’m doing a tea related business, and want to learn more about tea and the water temperature. I can say that you can drop about 10 degree C (18 F) by transporting hot water from a vessel to vessel. But it is just a very rough guide, and the results are very much depends on the situation, especially following the three elements.
1. Room temperature
2. Amount of water (servings)
3. Tea wares (size, figure or material)
So, I cannot give you one simple answer to this issue. But I can show you some examples on the next post, and I’ll give you a tip in the end. Have a good weekend!