As the impact of currency intervention by authorities fades, a senior strategist at the Bank of New York Mellon has turned to the price of a popular Japanese fast-food staple to accurately gauge the yen's weakness.

Geoff Yu created the Curry Cutlet Index as an alternative to The Economist's Big Mac Index. He believes comparing the international price of Japanese curry cutlet rice better reflects how yen depreciation feels to the Japanese public, compared to the Big Mac burger.

On Wednesday morning in global forex markets, the dollar was trading around 159.23 yen. However, according to Yu's calculations, after considering the yen's purchasing power reflected in curry meal prices, the dollar should be worth only 62.18 yen. This suggests the forex market is significantly undervaluing the yen. By contrast, the Big Mac Index, based on the global prices of McDonald's burgers, indicates the dollar should be valued at 80.30 yen.

Yu said that if the goal is to align purchasing power with high-income countries, this index shows the yen needs to be much stronger. He based his calculations on the prices at CoCo Ichibanya, the world's largest curry rice chain, which operates around 1,500 outlets globally.

After the United States and Japan arranged the most notable intervention in 15 years, pulling the yen back from 40-year lows against the dollar, the yen became the focus of global currency markets. But half of the gains from this historic intervention have already evaporated.

With the yen at historic lows, overseas travel and buying foreign goods have become staggering expenses for many Japanese, while their wallets are also pinched by rising domestic food, service, and consumer goods prices.

Yu said that if the price of curry cutlet or ramen in Japan becomes unaffordable, calls for a policy shift are likely to grow louder.

There are other alternative indices based on purchasing power parity. For instance, the Latte Index tracks the global price of Starbucks coffee, and the KFC Index is designed to better reflect purchasing power parity in Africa, where the Big Mac burger is far less common.