The description of this tea makes it sound a little unusual due to the "old growth" tea trees in Yunnan. Sounds like it may be a green made from the Assamica varietal like the puerh teas. But I don't think that would change my approach to it as a green.
With a new green I usually start with cool water, 160° or even less, 3.75 gr in a 6 oz mug, 1 minute, and feel my way from there, increasing time and temp each subsequent infusion up to 4 or 5 infusions and sometimes even boiling water.
As you have pointed out, tea leaves come in all kinds of shapes and many don't fit that well into a spoon, but I tried weighing and measuring appox equivalents. One-third gaiwan still looks like too much to me. I would start with less than a quarter. I found that 1 very heaping teaspoon (measuring spoon) was about 2 gr therefore about right for a 100 ml gaiwan. I don't find it so easy to judge % fill of gaiwan, but this looked less than one-quarter full.
Course, in all things tea, you will likely change your parameters as you get to know the tea. The first time is mostly experimental. Just as in love.