1. A method for controlling an emissions gas reduction with a Selective Catalytic Reduction (SCR) device, comprising: monitoring operating conditions of the SCR device, wherein the SCR device is supplied with a reduction agent; obtaining an operating time and an operating temperature of the SCR device; determining a target emissions gas conversion ratio of the SCR device based on the monitored operating conditions; determining an injection amount of the reduction agent to the SCR device based on the monitored operating conditions; and supplying an injection amount of the reduction agent to the SCR device based on the determined target emissions gas conversion ratio and the monitored operating conditions.