The Margin-Level article talks about 100%. My first question was basically, if I have to replace that 100% with 80% at Kraken? (because it's written somewhere that Kraken uses a 80 Margin-Call-Level)
I see that the other questions were not answered by the links, but your first one is. Margin call is 80%. 100% is the level at which you cannot open new positions but that is not margin call. "If my leverage is 2:1 (and I don't have more balance) the price only has to move 40% (instead of 50%) in the wrong direction to be margin called?" - this cannot be answered because your margin level depends on your account equity and the margin used as explained in the links I provided. You cannot choose what currency will be used as "collateral."
So open questions:
1) Do I have to replace that 100% with 80% at Kraken? https://support.kraken.com/hc/en-us/articles/203325753-Margin-Level
2) If bitcoin is used as collateral, will it be used as " 1 BTC " regardless of the price? Or will my collateral loose value (in addition to the normal loss!), if the price of bitcoin falls.
3) When I open a short position and I have Euro and BTc in my account balance. How to determine which currency will be used as collateral (since both can be used)?
1) No - answered above I think - margin call is 80% and the level at which you cannot add new positions is 100%.
2) If the price of bitcoin falls, then the value of your collateral also falls.
3) The system doesn't assign a certain balance in a certain currency to the margin position - it is more flexible than that. Suppose you have 1 BTC and 400 EUR in your account and go long 0.5 BTC @ a price of 400 EUR and 2:1 leverage. The margin used to open the position is 100 EUR since you used 2:1 leverage. Assuming for simplicity no change in market price, you could then withdraw all of your BTC or all of your EUR because either way the remaining balance in your account is sufficient to keep the position above 100% margin level (we don't let you withdraw below 100%). Either way your margin level is at 400% because your equity is 400 EUR and your used margin is 100 EUR and margin level = equity/used margin.
Hope this finally answers your questions!
Mister Dargo, excuse me, can you check our request in private messages? We are little bit confused cause there are no answer about our verificaton and payment.