Abstract: A method is provided for calibrating an after-treatment system retrofitted onto an engine, the after-treatment system including a Selective Catalytic Reduction (SCR) device therein. The method is implemented using a controller having a memory pre-fed with an Engine-out NOx map, a SCR conversion map, a maximum SCR capability map, and a NOx dosing map corresponding to bench-run conditions for the given engine and the after-treatment system configurations. The method includes updating the Engine-out NOx map, the SCR conversion map, the maximum SCR capability map, and the NOx dosing map by the controller on the basis of actual sensed parameters of an exhaust stream associated with operation of the given engine and after-treatment system. The method further includes directing the controller to vary a feed-forward reductant dosing quantity at the SCR device during a subsequent operation of the engine based on the updated NOx dosing map stored at the controller.