The margin depends heavily on what other positions are currently open in the account. It looks like domestic had mostly short calls still open. So the margin required to enter the short puts was probably $0. IB has a demo trading feature. You might check into that. Then you could start with an empty portfolio, papertrade the short calls (or whatever), and see the margin required for that one trade. Then add some short puts and see how the margin changes (or doesn't) when you do the new trade.
ib's system imo is very clear and understandable...now my journal, may not be. i knew that in the beginning, so i am evolving to try and be more clear. it is fasinating to see how the values of short options change, i am hoping to continue to illustrate this through my postings.
i think it is important to try to show what has changed in my account as the er2 index has risen recently(763,roughly). i am short: -10 820/815/805 each calls november i have 35,000 liquidity overnight(which is the only important measure). when the er2 was at approximately 745, my liquidity was at 56,000.
when the underlying reaches within 25 points of the short option, i will sell a following month option higher(or lower if a put) for the equal or better amount of the short closer month option that i have to buy back. an example would be(using oct even though i do not have any short october's): er2 @ 767.10 my short call @ 795 /.45 ask nov 830 @ .65 bid. .....my new position MAY now be nov 830. i also may go higher or lower.doing this is not an outright loss in my book, especially when the option that i am forced to buy back is for less than i sold it for...or we just may see an actual roll....... fyi , i have done this before , i just prefer not to of course. i might also add; those who do spreads either ctm, or otm have already taken losses , i have not, although i am 15 points away from a "roll".