Hi there, your initial margin is essentially what you need as a minimum in your margin account to establish the trade/position. The maintenance margin is the point at which you need to top up your margin account.
In this example the initial margin of 65% would mean that you need to have 65% of 500 x 75 in cash in the account. This is 24,375. Given that the total position is worth 37,500, the broker with cover the 35% position or 13,125 and hold the securities as collateral. But to add further insurance and in order to protect this money the broker requires the initial margin to be restored if it falls to the the maintenance margin level. The maintenance margin is 55% of the initial total position (500 x 75) 37,500 which is 20,625. Essentially this is saying that the equity component on the trade is 55%.
In order to work out which price the 55% equity corresponds to you anchor the borrowed amount 13,125 as 45% of the new position (after the price fall) which implies to overall position is 13,125/45% =29,167. This gives the equity portion as 16,042. To work out the price of the stock under this condition you divide this total new amount of the position by 500 (the number of shares). Which gives 29,167/500 = 58.33. So if the shares fall from 75 to 58.33 you will receive a margin call.