I do not know that system but I suspect the errors in reading may be due to a basic design question?
If you place a sensor on the bottom of the tank, what does it read? I might guess it read the amount above the sensor? Correct me if not right on that!
But my theory is that it is having trouble reading the total volume when the height of the contents changes.
The problem is that the tank is not just a rectangul but has a sloping floor. A one inch dpethy at on point will not be the same reading doubled as the contents get deepr but also wider!
That may not make sense but looking at the shape of the tank makes me think reading from below is not going to work when the floor slopes?
I actually see much of the reason for any sensor being prone to being off at some point, so calling out "about 1/3, about 2/3, is about as good as I see happening!
At this time, I see it as two options. We can have a tank with a simple shape to read better or we can have a tank with the max volume in the given space and which drains well.
But It seems we can't currently have both!
I find the sensors do work better than they used to but not much!