Quote from zdreg:
if maintenance requirement is 25% spy can drop 33% before receiving a margin call on an overnite position.
e.g. purchased 100k spy with 50k of your own money.
if spy becomes worth 67k your equity is now 17k (50k- 33k loss) and the stock is worth 67k. 17k/67k = 25% if it moves lower u receive a margin call.
Maybe this could be handy: 33%=[1.33]-1=[1/(0.75)]-1=[1/(1-maintenance)]-1
One of the next questions is: why did the broker choose the 33% mark, but not another number?