UOM coneversion factor

I customize Sales order as per client requirements and put custom field conversion value to take input and calculation be like:
qty as per stock uom = (quantity * (1/uom conversion value))
66 = (3808 * (1/56))
66 = 3808 * 0.017857143

In delivery note:
same be like
client wants to give discount on sales quantity
qty = 3524.4 and i have change conversion value so that conversion factor will be changed and based on that nos will be calculated.
calculation is okay but it shows validation for convesion factor from sales order.

Please provide solution for this.