I don't know the answers but I think these are exactly the right questions to be asking and seeking your answers to. These are the questions that transition this activity into something you can depend on.
Hmm... focusing on put spreads I'm wondering, what if I set my margin fraction so with unused margin I could "pull the ripcord" and buy back all short legs at, say, double their current price with margin as it would be if the stock price was $75 lower. Or maybe triple and $100 lower. Something like that where I could be pretty confident of getting out of a position that's turning bad.
Unfortunately this isn't that easy to capture in a single calculation, though it should be fine in a spreadsheet.
But it does highlight the leverage dangers of e.g. shrinking $100 spreads to $50 spreads in order to open more $50 spreads -- because that doubles the number of short legs and makes the ripcord a lot more expensive. That expansion in total # of positions definitely starts to look like something that should only be done during really positive stretches. Last week I did well with that, but now I'm thinking I better see how the week opens before trying it again.
Finally, on a side note, I've been tracking my margin and the "SMA" it's based on, and I think I've figured out that (at E*Trade) today's SMA is simply half my total share value as of yesterday's close. I've not been able to get it to match exactly for a lot of days in a row, but it's been within half a percent or less. If that's correct, I don't know why they can't be bothered to just say so, instead of only saying "today's SMA is based on yesterday's SMA altered by x, y, and z". That's the value I'd use to calculate the decrease in usable margin as the stock price goes down, since as far as I can tell the E*Trade margin calculator only lets you see the effect of proposed trades, not the effect of changing share price of existing holdings.
Edit: and SMA updates once a day, at some point after close.
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