that site is terrible for estimating fees.
it doesn't apply here anyway since bitmex doesn't allow you to choose a fee rate (satoshi/byte). your only choice is to set a flat fee above their minimum.
Where can you find the recommended tx fee?
Be good to know that as and when.
basically all you can do is set it to
bitmex's recommended fee (or higher) and hope for the best. the way they handle withdrawals is incredibly dumb. it's silly they even ask customers to input a fee amount since the transaction may be broadcast many hours later under different network conditions, and we can't see how many inputs they will use!
they don't batch tx either and are comically bad with fee optimization:
https://twitter.com/ziggamon/status/1134490591264497664