Isolating the gain that came from the rate
Multiply the amount by the difference between the buying and selling rates and you have the currency gain or loss. Buying a dollar at 1,300 won and selling at 1,380 earns 80 won per dollar, or 800,000 won on ten thousand. The return is that figure divided by the purchase cost.
Only the exchange-rate component appears here. Interest on a foreign-currency deposit and price movement in an overseas stock are separate, and real performance combines them. A share price can rise while the currency falls further, leaving a loss in won.
Enter rates that already include the spread you actually paid on each side. Using the mid-market rate on both produces a gain that could never have been realised.
Tax and costs belong in the picture
In Korea a currency gain on a foreign-currency deposit is not itself taxed as income. Interest on that deposit is taxed as interest income, and gains on overseas shares fall under capital gains tax, so the treatment depends entirely on where the gain arose.
Ignoring the round-trip exchange cost inflates the result. A spread is paid on the way in and again on the way out, so a rate that returns to where it started still leaves a loss. At a 1.75 percent spread, nearly 3.5 percent has to be recovered just to break even.
A plan that waits for the rate to recover has no deadline attached, which is its weakness. If the money is needed on a fixed date, the rate on that date is not something you choose, so funds with a near-term purpose are safer kept out of currency exposure.