Currency conversions of trading costs as well as profits and losses from trading activities are done using the prevailing mid-spread close rate as of 17:00 New York time, plus/minus 0.5%. For FX Options the rate is plus/minus 0.1%.
The Currency Conversion fee does not apply to margin collateral. Only settlement of actual payments to or from the trading account are included, for example, buying/selling cash Stocks, paying/receiving options premium etc.
The rate used for currency conversion of amounts booked to your account is shown in the trading platforms under the “Trades Executed” report.