DistanceError

Hi;
I am using MDEk1001 modules and I currently measure the error in raw ranging measurements from the serial port. I want to find a relations between the error in ranging, for example if i am 1 meter closer to the anchor, the error is 10 percent lower. However, I cannot find this value, the errors are changing in the same condition and not always longer distance equal to more erros. Seems like in the middle is lower probably!
Anyone has any idea???

Thanks