How to enter fluctuating values in a calculated control?

[rowan1]

I have a caculated control on a sub form that converts my purchases from the currency I buy in to the currency I sell at.How can I alter the rate I convert at to allow for fluctuations in the exchange rate without altering data already entered? My calculation is

([unit price]*[Quantity])*exchange rate

 

[RichUK]

Store the exchange rate at the time of the purchase