Years 1 and 2 I used the elevator's scale, years 3 and 4 I used portable axle scales (https://www.scaledynasty.com/Portable-Truck-Scales-p/ps60kww.htm).  This year (year 4), each weighing was of similar weight (same wagon, similar pounds grain in the wagon for each cal load, wagon then emptied after each load), to eliminate the possibility of the scale calibration being part of the problem.  Year 3, we dumped a calibration load in the wagon, weighed it, then tared the weight, then dumped another calibration load in the wagon and weighed it.