OK, they added a volume to use for that amount, 80 ml. This is a pretty rich ratio of more than 3:1. If you try this with a mediocre Gyokuro you will be sadly disappointed.
Alternately to using more water and thus much more gyokuro if you are following a set ration, you can use smaller cups or simply pour less tea into each cup. This makes preheating even more critical IMHO.
Ippodo wrote:The key to preparing Japanese green tea properly in a kyusu teapot is the combination of three things - the amount of tea leaves, water temperature, and brewing time. Firstly, the most fundamental aspect is the amount of tea leaves used. It is important to note that the proper amount differs depending on where the leaves were grown and the tea dealer. The proper amount that Ippodo specifies for bringing out the taste that Ippodo wants its customers to enjoy is 10 grams.
Notice the 3 important things they list ...
Amount of leaves
Notice the omission of amount of water ... perhaps this is implied? But they repeatedly stress using 10 grams of leaf. This confused this Westerner. Instead of amount of leaves, I would personally say the Leaf to Water ratio ... IMHO.