For the BPS margin in an IRA type account, the margin is easy to calculate - 100* spread width per contract. So
@mongo example - that's an $85 width and thus $8500 margin per contract. If you're doing 100 of those contracts then you'll need $850k in margin. What I actually see in my IRA accounts is that this 'margin' shows up as a Pending Transaction. So if I had $1M cash in that account and did this trade, then my cash would decrease by $850k (down to $150k) and my account would get a new line "Pending Transaction" for $850k. That line will stay there until the trade is closed later. That Pending Transaction line will also start with the premium received on open - that'll take 1 or 2 days and turn into incremental cash on the Cash line, leaving the $850k as a Pending Transaction.
In my brokerage account where I have actual margin, I do the math the same way. Even if my broker (Fidelity) would enable me to do that $850k transaction using (for example) $500k in margin, I would still mentally set aside the whole $850k (cash). My rationale is that I'm already getting a lot of leverage (as much as I want in fact) within the BPS - I don't need my broker extending a bit more. I haven't put any effort into selling BPS using margin on TSLA shares - somebody else will need to address that.
The issue that
@ReddyLeaf identified is a real one. It doesn't change the margin requirement but it does illustrate a case in which you can dig a really deep hole for oneself. These spreads need more attention than buy and hold shares or purchasing long dated calls. I don't know the mechanics on what the broker would do in this circumstance. My guess is that over that weekend the 685 put would be assigned and you'd buy 100*# of contracts worth of shares and pay 100*# contracts*$685 for them. Of course in this example you've got nothing like that level of cash so you'd also have a margin call that is large enough I would expect your broker to sell all of those shares first thing on Monday, and you just get to hope that the open is higher than $685.
The lesson that I take away - don't take a planned absence with BPS open. Period, full stop. The other lesson that I take away - I would at least like my wife to know enough to close up our BPS should the need arise. More generally having a backup plan if possible that isn't dependent on oneself to close already open positions.