bamboo wall thickness forn1.8 No-Root shakuhachi

hi from Italy,
I'm working some bamboo that I've found near the river in my town in order to manufacture a NO-root shakuhachi, I decided to try a No-root version since i don't want to extract a live bamboo, I'm only using the ones already avaiable in the ground, I've some bamboo with a wall thickness of 7, 5, or little less 4 millimeter, the diameter of theese bamboo are from 3 to 4 cm
so my question is:
which is the preferred thickness for a 1.8 shakuachi?
I manufactured 2 of them using a diameter of 3.5cm circa the bore then is influenced by the thickness I can ear a different quality of sound
which thickness do you suggest to use and look for ?
thanks
Luca