here is one good practical way to determine stall speed
1- fly to a safe altitude
2-switch to FBW-A or Stabilize mode
3- fly level
4-decreasing throttle try to reduce speed
5-almost reaching the stall speed airplane will oscillate around…"
"I had the same problem during my setup in my ground tests but in flight such thing did not happen. (I almost took a risk). at that time I thought it is matter of speed. now it seems it is not! now it is more risky. I had arduplane 2.72!