AAPL trade = more trouble than its worth

I got some clarification on how margin works from Interactive Brokers and from reading around on the web. I don't exactly understand why my margin ran out even though my account had excess liquidity, but the important thing is that I know how to know if it is going to happen again. Apparently I ran out of "Regulation T" margin for establishing new positions. This check is made 5 minutes before the closing bell by Interactive Brokers. I nievely assumed that if I opened my account window and the excess liquidity it showed was greater than zero I had plenty of margin to open new trades. Apparently this is not the case. I need to check another field called my "SMA" account. I don't really fully understand how this is calculated, and I don't care really. Due to my extreme distaste for losing large amounts of money I rarely use anywhere close to the max margin. The important thing is that I now know that if the SMA value is negative then I'm going to get a margin liquidation. I'm surprised this has never been a problem before. This was my first margin call/liquidation ever.

I closed out the AAPL positions today. The trade wasn't working, and it was using up tons of margin so it seemed prudent to take my losses now rather than try to keep working it to try to reduce losses further or get a profit from this point considering how much capital was required to keep the AAPL common short open. It's pretty stupid that a position that has the exact same risk as a put option (short stock, long a call) that can be purchased for a few bucks requires the same margin as a shorting a share of common stock. I know there is some move to switch equity options over to SPAN margining which is what is used in the Futures market. Maybe that will be addressed then, although I really don't know the ins and outs of SPAN margin calculations. I was able to narrow my losses slightly. I did manage to get out of the position with only a 66 cent loss instead of the 84 cent loss I was running yesterday. If you count the gains I made from my forced DJ position liquidation yesterday the loss was only 40ish cents. I'm going to count the DJ liquidation gains as part of this trade since the liquidation was caused by shorting all of those AAPL shares.