A method for controlling dosage of a reagent in a selective catalytic reduction (SCR) system includes the step of providing the system with a plurality of reagent injection nozzles, each nozzle being configured and adapted to be independently calibrated during an SCR reaction when the system is in operation. The method further includes determining an influence coefficient for each injection nozzle for a catalyst independently of the other injection nozzles, and optimizing the flow of reagent from each injection nozzle to minimize a sum of deviation across a surface of the catalyst. A system performs selective catalytic reduction (SCR) and a machine readable medium contains program instructions to controlling dosage of a reagent in a selective catalytic reduction (SCR) system.