Even if you miss some centimeters or having it not exactly balanced on the same level, the theoretical fault you make, is neglectable and falls problably within the inaccuracy of the scales
If you question the accuracy of a measurement, it's very easy to test this:
roll the trailer 1 meter further and start the measurement again starting with the last hull.
Check the difference with the first measurement. If it's too big, than I eat my shoe
I agree that it is about what your tolerances are.
If you are satisfied with + or - 5 pounds (10#), it is negligible.
More important is to calibrate your scale. Bathroom scales can be off by 10# at 200#. If you calibrate that same scale it can repeat within a pound or two.
