This can be better explained with an illustration. For example lets say you have the following:

     a. MTF cash collateral - Rs. 826262

     b. MTF stock collateral (after haircut valuation based on previous day's closing price) - 4813142

     c. Margin Required on funds utilized (basically the funded stock position) - 3209244

     d. Other loss (which is booked loss and charges) of 196273

     e. Mark to Mark loss - 2171604

     f. Accrued Interest - 17378

In the above scenario, the formula for calculating the margin available is 

Cash Collateral +Stock Collateral - Margin Required - Other Loss - Mark to Mark loss - Accrued Interest

826262+4813142-3209245-196273-2171604-17378=44904 Credit which is the margin that is available to take further MTF positions.

As we fund a maximum of 50%, you may buy further for a maximum value of Rupees 89808 under MTF.

The Margin section of the MTF statement emailed will look as below