I don’t understand why margin required for the trade is not used in the calculation…
There is no margin required for cash-backed put writing, as such.
The cash pile provides all the security you need. You aren’t borrowing anything.
It changes a bit if you use a bit of leverage: more aggregate assignment value of your puts than cash on hand.
If you write puts with an aggregate notional value somewhat higher then your cash pile, you are tying up some of the marginable value of your (all cash) portfolio.
i.e., you couldn’t use the marginable value of that cash for something else while you have those short puts open.
But you’re still not borrowing any money, and not incurring a margin loan, so there is no cost for it.
If you write a naked put WITHOUT a cash pile to back it up, well, that’s a horse of different colour.
Since nobody sane does that, it’s not worth calculating the margin requirements of it.
Jim