Clearing House Margins is quite a complex and confusing topic. Every night the ASX publishes the closing prices of all option series traded on the ASX. This amounts to about 20,000 lines of data. Of course, to calculate an option price, you also need to calculate the Volatility of that particular option code. So the ASX also publishes their estimate of this number.
What I am trying to demonstrate with the above chart is that the ASX estimates vary wildly between the strike prices of a Stock that have the same maturity, in this case, ANZ July Options.
Hopefully the chart demonstrates that this is not an exact science. The July $32.00 call volatility is published at 2%. The July $32.00 put volatility is published at 32%. Now you might think that because it is an out of the money option, the price difference in the values is not that great. The problem is though, when the Clearing House calculates margins, it adjusts the share prices up or down by about 5% to 30% depending on which stock it is. ANZ's price is moved 8%, whilst a stock like AWB is 31%.
So Margin prices can vary greatly, and therefore so can margin calls. This is why sometimes that if you have a credit spread with a maximium risk of $5,000, the margin call could be over $5,000 (greater than the maximum risk).
We manually calculate margin calls that calibrate with the Clearing House margin calls for each account. This allows us to pinpoint which of your option holdings are requiring the greatest margin. I don't know of any other option broker that can breakdown a margin call into each their different stock components. So remember, if you have any queries on margin calls on your account, we have the information here to help you better understand this area.
We are able to supply an excel spreadsheet that calculates your option margins, which includes all margin offsets should you have a portfolio of options. It would help with "What if" scenarios. If this is of interest to anyone, could you please email Gary or Vince or any of the brokers to see if there is enough interest to make it available on a daily basis.
We often have queries about how Stock volatility is calculated.