A margin call on a short position (you are selling shares you borrowed from the broker) is received when the price of the underlying is going up instead of where you hoped would go, and that is down. This is because you will be asked to either return the asset (that you sold without having – you borrowed it from the broker) or pay the difference between your margin account and the market price of the underlying. Say you short the underlying at 50, if price goes up to 60, you would have to deliver the underlying of 60 or pay the difference: sold @50, bought back @60 to return it. NOT THAT GREAT!
The current price of the underlying is X (that we will determine soon). For the argument sake we shall consider that you short one share.
At the time the stock was shorted, you had to deposit a margin of 40% of the asset: 0.4*50=20 . Proceeds form the sale of the underlying are deposited into your margin account. The initial value of your margin account becomes 50+20=70
Daily, the account is marked to market to account for gains or loses. At all times, your margin account has to have at least 30% of the current market value of your portfolio, or you will receive a margin call.
Margin = (Initial margin account– Current value of your position)/ (Current value of your position)
0.3=(70-X)/X, solve for X, X=70/1.3=53.8462
Hope this helps. Myself, I cannot memorize formulas, if I don’t understand the concept.