I am looking at the documentation (OptionMarginModel and ISecurityMarginModel) and am having trouble understanding how I can set a custom margin model for short options. Right now if you short an option it appears as if you need 80% of Underlying's Market Price. However, the IB Formula is only approximately 20% plus the option price. I set the brokerage model to InteractiveBrokersBrokerage but the margin for short options is still ~80%. The problem here is that my backtest's orders are being rejected due to insufficient funds even though in real trading it would still be permitted.

How can I create a custom margin model for short options?