Changed in myeongdong at 809 when in SG highest showed 807.
I always check SG rate at
https://cashchanger.co/singapore/sgd-to-krw
Anyway, not all Myeongdong money exchange rate are good. Only the one opposite cafe 1896 / Chinese embassy red gate which give better rate. There are other money changer around there, there's 2 red gate of the Chinese Embassy. Don't approach the wrong one.
That day when I changed 805, the other money changer in front of the other red gate was offering 800, and while walking around, saw other exchanger showing 790.
Walk a little bit to get a better rate. Especially if you're changing big amount.
Make sure first stop in Seoul is the Myeongdong
Out from Myeongdong Subway station exit 5, walk along migliore till junction where you'll see C&U, turn right into myeongdong 2-gil. Walk past myeongdong 8na gil, the second junction you'll see the money changer at the corner like below: