Both maximum displacement and displacement ductility factors should be considered in the design of a blast-resistant structure since both parameters correlate with an expected performance level of a reinforced concrete RC structural member during a blast event. The blast-resistant design procedure discussed in this paper takes into account both the maximum displacement and displacement ductility responses of an equivalent single-degree-of-freedom SDOF system, while the response of the SDOF system is made equivalent to the corresponding targets of design performance. Some approximate errors are present when comparing the actual responses of the structural member, which has been designed for blast loading, and their corresponding design performance targets. Two indices are defined to quantify the approximation errors, and their expressions are obtained through comprehensive numerical and statistical analyses. By using the error indices, the design procedure is then modified such that the approximate responses of the RC member are equivalent to the targets of the design performance. The modified procedure is implemented in three design examples and numerically evaluated. It is concluded that the modified procedure can be used more effectively in order to ensure that the actual responses of designed members reflect the respective targets of design performance.