This is irrelevant to margin calculation. If I make $2, it's 25% profit (2/8). If I lose $2, it's 25% loss. If I lose $4, it's 50% loss. If I lose the whole $8, it's 100% loss.
Lets take as an example IC with $2 creadit.
In order to lose 50%, the spread in money should be worth about $6...