Which currency to use for mark to market?

Just finished a question for mark to market in reading 12 for currency exchange.

It found the market to market by finding the net of the first forward position and what I think is like a second hypothetical forward position. In the question the person was selling USD for JPY and is marking to market at 6 months prior to settlement date.

The answer has the net in JPY (however many million USD x initial forward JPY/USD - however many million USD x current 6 month forward JPY/USD, then discount this back by JPY 6 month LIBOR). My question is, why is the mark to market expressed in JPY instead of USD? How do I know whether to use the USD or JPY LIBOR? Is it because the person is holding JPY at the time of the mark to market?

The mark-to-market is based on the currency you’ll be paying.

I wrote an article on this that may be of some help: http://financialexamhelp123.com/mark-to-market-value-of-a-currency-forward-contract/