Containment flooding is an important strategy for severe accident management of a conventional boiling water reactor (BWR) system. The execution of containment flooding requires information about the water level in the primary containment. However, there is no instrument to measure the drywell water level for most Mark-III systems. Furthermore, because of the design feature of the Mark-III containment, the water level in the containment does not necessarily guarantee that there is an equivalent water level in the drywell. Therefore, the development of a drywell water level computational aid becomes very useful. The purpose of this work is to develop and validate the drywell water level computational aid and to investigate the implementation of the proposed computational aid on the containment flooding strategy of a Mark-III system. The Kuosheng nuclear power plant (NPP) is a typical BWR-6 NPP with Mark-III containment, and the Severe Accident Management Guideline (SAMG) of the Kuosheng NPP has been developed based on the BWR Owners' Group Emergency Procedure Guidelines and Severe Accident Guidelines, Revision 2. Therefore, the Kuosheng NPP is selected as the plant for study, and the MAAP4 code is chosen as the tool for analysis.