I'm working on my own option trade calculator. Once completed I'll put it online for everybody to use. Part of that is to calculate margin on sold options (single or as part of spreads, synthetics, butterflies).
The ASX provides a Margin Estimator with an awful interface. It says there it uses TIMS to calculate the margins.
Does anyone know the formula this TIMS thingy uses to calculate option margins? Mind you, I found a page where one can download a file containing different margin parameters (such as margin interval) for each stock. So, once I have the formula I should be able get the same results as the ASX Margin Estimator.