http://www.interactivebrokers.com/en/trading/marginRequirements/currency.php?ib_entity=llc May be worth a look if you haven't seen it already. I didn't see a Yen example though.
Assume IB account is in base USD When you load into TWS, If you trade GBP, you are loading the GBP amount, when you trade JPY you load the JPY amount ( hence you are not loading the USA amount ) Correct ?
Finally IB Chat was useful.. When you select a cross rate in TWS IDEAL PRO Cross rate UnderLying GBPUSD GBP JPYUSD USD The QTY you load into the TWS to trade in the BID/ASK area is the Underlying value...no matter what you see in the QTYs in the Bid and ask fields. The underlying symbol determines QTY value to trade. Example : So to trade USDJPY with underlying of USD, to BUY 45,000,000 YEN @ 117.67 you load into TWS QTY USD 382,425 dollars. Margin however is always calculate on the USD portion of the cross. Do you concur...