Typically the manufacturer specifies the value for K. (30 is common for cylendrical rotor machines). The curve is drawn for t = K/I2^2. I2 is per unit of nameplate current. I don't know if that answers your question or not.
The ability of a generator to accommodate unbalanced currents is specified by ANSI C50.12 and ANSI C50.13 in terms of negative-sequence current (I2). IEEE C37.102 specifies the continuous I2 capability of a generator and the short time capability of a generator, specified in terms I2[sup]2[/sup]t and defined in a dual-slope relationship plotted against generator MVA.