trying to understand margin calls

Could someone explain how much money must be placed into an account if a margin call is triggered.

Say I have 1,000 shares @ $10. Portfolio value = $10,000 with a margin required of $1,000 (10% margin rate) to take up position.

Now say the price drops to $9 and the total portfolio value becomes $9,000. What would the margin call be. Would I have to place into my account an extra $1,000 to cover the difference or only 10% of the $1,000. Or is there another formula.