You need to include more information. There are different factors that will come into play with determining the correct height. Weight of item being picked. Geometry of item being picked and how you are picking it. Example if the item is normally 2-3 feet high and you are picking it up from the top the conveyor would need to be low to the ground(ex. luggage belt at airport terminal). If the package is smaller < 1 ft high then you need to keep the operator/assembler from repetitive bending. The guidelines are very much dependant on the application. You might want to consider layout of the workstations in a drafting package to determine the appropriate elevations.
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