The 1st leg order will be a market order. The price considered for computataion of required margin is the weighted average price of the best 5 bids (in case of sale order) / best 5 offers available (in case of buy order). 

Margin amount = {(Weighted average price of 1st order - limit price of cover order)* Quantity of shares} + {(Weighted average price of 1st fresh order * quantity of shares) * cover margin % for the stock}

For example, you place an order for buying 200 shares of Axis Bank. The top 5 offer prices are as below.  The weighted average price will be calculated as:

Best 5 offers