Not sure if I understand what you are saying , you put a 20 kg bag in the middle of the scales and it reads 28kg so you turn the adjuster until it says 20 kg , when the scale is empty it doesn't matter that the needle is not at zero , if the adjuster is at maximum and still not right then remove the tiny nut holding the needle , pull of the needle and realign pointing to zero after centralising the adjuster . You could take the back off the scale and clean /lubricate , they are simple and very robust my set is 40 yr old and going strong