When the margin level ratio drops to or below 20%, the account will be stop out.
Margin level = Equity / Margin
For example:
Let's assume your trading account currently has 1,000USD and account leverage is 500:1. Then buying a lot of XAUUSD priced at 2,300. At this point, your margin will be : (2,300 × 100) * 1 lot ÷ 500 = 460 USD
Assuming a loss of 20USD, at this moment, your margin level will be
(1,000 - 20) / 460 ≈ 213.04%
Now, assuming your equity reaches x when a stop out (20%) occurs, your equity at that point will be:
x / 460 = 20%, x = 92 USD. Therefore, when your equity falls below 92USD, a stop out will occur.