I have a portfolio margin account and have not figured out how IB is calculating margin requirements for BULL put spread strategy. Any thoughts?
No. However, it will be a considerably easier exercise for you to apply a 10% margin ratio than 50% margin ratio on a conventional account as an approximation. There is a considerable uphill battle in applying those PM "Scenarios". And if you really require that rigour to avoid liquidations, you are playing a very dangerous game.
Back to the bull put spread question: Say I enter leg 1 of the trade today (sell @10) and Leg 2 next week (buy @ 9) would those 2 trade match automatically on the day of the second trade to net-off (reduce) my margin requirements? or do they need to be entered simultaneously on day 1 as a combo order?
They definitely should. Sorry to sound cryptic. And, watch that "Margin Impact" section on the IBKR Order Preview. I don't recall them not applying it properly. But, my trading is very simplistic. Risk on/Risk off.
It depends on what you’re trading, your overall risk, how it’s correlated to SPX, how much concentration you have in a single stock, etc. All this is recalculated nightly. Some calculations are proprietary to IB and no one succeeded in obtaining full math from IB, especially as IB changes their own calculations as well, especially when they detect higher risk across the board. This is what makes IB the most advanced and safest broker to park money with. Here is some info from IB on regular margin requirements on various option combos, and then on how portfolio margin works in general: https://www.interactivebrokers.com/en/index.php?f=26660#portfolio-margin-page