How it works
KRW = foreign amount x rate. Buying adds the spread to the rate; selling subtracts it. Set the spread to 0% to use the published rate.
Where the rate comes from
The Board of Governors of the Federal Reserve System publishes these rates on a New York basis (public domain data). They differ slightly from the Seoul close and from bank counter rates, and there is no new value on weekends and holidays.
What to keep in mind
Actual spreads vary by bank and by any preferential rate you have. Overseas stock trades are usually settled at the rate on the settlement date.