The hedged margin is the security to open and maintain locked positions required by the broker. It is fixed in the contract specification for each tool.
FBS has a 50% margin requirement on the hedged positions.
I.e the margin requirement will be divided among the two positions: 50% of margin for the orders in one direction and 50% of margin for the orders in the opposite direction.
Some brokers have no margin requirement, but that leads to a situation when some traders open disproportionately large positions compared to the size of their balance, because when the price moves, you are down on one of the positions, but up on the opposing one for the same amount, so your profit equals your loss until you close one of the positions. Due to this, some clients received margin calls when closing one side of the position (which triggered an added margin requirement for the remaining un-hedged side).
The result of the hedged positions seems fixed, however, it varies together with the spread – so a sudden spread widening (let’s say during the news release) can also lead to a margin call.
Margin (Forex) = lot size x order volume / leverage
Margin (CFDs, Metals and Stocks) = opening price x contract size x order volume x margin percentage / 100
Since the margin considers the current price, if the spread widens, the price will change as well, thus, the margin level changes too.