Understanding the optical flow quality measure

Hi everyone,

I’m making my own drone using a custom firmware. I use a SLAM camera to get different measures like position and velocity but it struggles a bit outdoors, so I am trying to add an optical flow sensor (the ThoneFlow to be precise) facing downwards to get another velocity measure when the quality of the SLAM is not good enough.
My problem is that I don’t understand the ‘surface quality’ measurement given by the sensor and I don’t feel I can rely on it to identify when the sensor is working properly. Here is the manual of the sensor but it only shows the output but don’t specify on how are these measures obtained.
I have done some tests and occasionally the sensor gives bad measures when the quality is at its max (255) or on the other hand, works perfectly when the quality is at 0. Leaving me with no way to know when can I actually use the sensor.
Has anyone had the same issue ? Do you know where can I get more information about the sensor like which optical flow method they are using or stuff like that ? Any ideas of how could I characterize the measurement quality without using more sensors to compare ? I would really appreciate the help.