Probably somewhere near 9wa shoe which appears to be around 3.5~4b - https://royals.ms/forum/threads/s-9wa-fs.195152/ & https://royals.ms/forum/threads/s-13wa-bfc-9wa-fs.194999/ If you're not interested in finishing it, some people trade their finished shoes for unfinished with 1-2 slots. Usually 1 slot = +1 weapon attack, so you could trade for 9wa shoes which might make it easier for you to sell if you decide that route.
