I have accounts at both Fidelity and Interactive. It appears to me that the margin requirements of the two are very different. Fidelity's are spelled out as the higher of (a)25% of the stock price minus any OTM amount plus the premium or (b) 15% of the strike price plus the premium. Interactive's aren't spelled out, but I used their demo ( the one that has Portfolio Margin -- which I have) AND FOUND THAT THE MARGIN REQUIREMENT AT I'ACTIVE SEEMS TO BE HALF THAT AT FIDELITY? Is this right?
You are comparing apples to oranges. What you wrote about Fidelity is Reg T margining, which is quite different from the portfolio margining.
Reg T vs. portfolio margin is hue. But, this may be the solution: Fidelity charges margin for the half of a butterfly (IB charges zero) that you sold. Fidelity charges margin for each half of an iron condor; IB charges for only one side. It's likely other are other differences as well. Mark