Purchase Requisiiton in Foreign currency

 How does one enter a foreign currency expense purchase requisition?  We are located in Japan, but our functional currency is USD instead of JPY.  When we create a Purchase Requisiiton, the default currency is USD, but we handle a lot of expense requisitions in JPY, the current process is to manually convert the requision into USD, e.g. for a 500000 JPY legal fee at an exchange rate of 1 JPY = .011837121 USD, we have to do a calculation and then enter 6918.56 in the amount/qty field on the line tab, and then enter in the currency tab we have to enter