What is rolling margin for reinforced steel and how it is calculated?

Rolling margin is percentage of diviation in Sectional weight of Reinforcement steel allowable as per IS codes.Reinforcement steel is extruded from a mould which is made for a particular size e.g 8mm Dia.When the mould is brand new,the sectional weight of 8mm steel extruded through mould would exact as per IS standard of lower than that.i.e. 0.395Kg per Metre or lessor.Mould gets little bigger after certain period of time or after certain quantity of production is taken from a particular mould.Now same 8mm dia bars extruded from the same mould will have more weight per Metre say 0.400Kg per Metre insted of 0.395 as per IS. That is more mass per Metre/Length is required for same length.This diviation in weight is defined in IS codes for different dia which is as under :- 8mm to 10mm +_ 7% 12mm to 16mm+- 5%. 20mm and above+ -3%. Rolling Margin is calculated as under :- Total Weight of Bars (Dia wise ) / Total Running Metre of Bars = Actual Sectional Weight of bars. Compare sectional weight with Standard IS Weight. Weight as per IS Standard.= Dia X Dia / 162.