At the National Metrology Institute of Japan, we are currently developing a transformer bridge circuit that can precisely compare the 'i = 2' plateau resistance of an ac quantized Hall resistance with a 100-kO resistance. In this report, we propose a practical transformer voltage ratio of 7.751, which is the most important parameter for developing a transformer bridge circuit. Further, we describe the design of an inductive voltage divider (IVD) to realize the 7.751 ratio. We intend to use the proposed IVD as a reference standard for calibrating the voltage ratio of the transformer.