So how does it work then for shorting? how do I calculate profits, liquidation, like deribit?
If you invest say 10 BTC in a short position, and price goes down 10%, you make 10% * 10 BTC = 1 BTC. If price had gone up, you'd have lost that same amount.
If you expect prices to go up, you go long; if you expect prices to go down, you go short.
The liquidation is the price at which your position loses 80% - that's when it is auto closed to cover your margin.