The margin on that CE is (4800+85)*50*12%=29310. This is the initial margin reqd. If the position goes against you, that is the market rises, then you have to have cash to provide for the additional MTM at EOD. if the market moves to 4800 and the CE becomes 165, then the contract value is (4800+165)*50. Margin is calculated on this at EOD and the difference between your initial margin and the margin at EOD is debited to your account. Similarly if the market moves in your favour, the difference in margin is credited to your account.