Cover Orders are only for option selling. Span + Exposure is charged on Option Selling but for option buying it's only premium amount (which is pretty small compared to Span + Exposure). There is no separate margin levied on a SL order.
So if you bought an 6000CE Option on Nifty, the price is around Rs. 200. One lot is Rs. 200 * 50 = Rs. 10000. An SL-M won't take up any margin so you'd be charged Rs. 10000.
Conversely, if you sold 6000CE Nifty Option, the Span + Exposure margin comes close to Rs. 28,000. The SL-M order normally wouldn't help you -- but with cover orders, the margin requirement comes down quite a bit.
SL range is 20% for option selling. Range for Rs. 20 option is up to Rs. 24.
Hope that helps.