Perhaps someone with experience RE: same can help. Does one gets 4:1 overnight margining for portfolio margining with Thinkorswim & other Penson brokers? Does this apply also if your majority of investment is in one stock?
Is your phone broken? Unlike IB you can call them and after getting in touch with the correct person they will explain it to you. IB likes to be black or white so they are relatively strict, while TOS is willing to work with you, within reason. FYI, portfolio margin is simply the ability for a retail broker to offer the OCC's TIMS based model for risk which is what floor traders get. For stocks that equates to a 15% move or 100/15=6.66 to one leverage maximum. Minimum portfolio margin, especially in a concentrated position, is usually insufficient to cover the broker's risk so each individual broker sets additional standards. That is why you need to call them and discuss specifics with them.