here's a website which has a "pocket size" conversion table 'cheat sheet' that you can cut out and carry in your wallet or purse...

choose the two currencies you want (JA dollar & US dollar), and click "Get my FX Cheat Sheet", and it will return a nifty little table that you can print and cut out and carry with you.

http://www.oanda.com/currency/travel-exchange-rates