They generally update the margin at 2 pm, so you see where the the market is at 2 pm, and then adjust accordingly for EOD.
Suppose the market’s at 18000 at 2 pm & you have a short straddle of 1000 quantities of 17000 PE & 19000 CE. Around closing, it rises to 18200. The CE side margin would have risen & PE side would have decreased. So, you can square off 100 quantity of CE & maybe add 50 on PE side.
It’s more of an approximation & takes a bit of time to get it right.
But an intraday margin change is a bit imp. for this & this is why I keep on harping about it on this thread, but to no avail…