Put the exchange rate into a global field (if you are a single user) or into a one-record preferences table. In the Invoices table, define the Rate field to auto-enter or lookup the current exchange rate.
Set it up so that this value is copied from your field into a different field referenced by the calculation each time you create a new record. We use this method on invoices so that price changes don't modify pre-existing invoices.
We use the auto-enter lookup field option to do this. An auto-enter by calculation option might also work for you.
Thank you for your answers. Sorry to sound really dumb, but how would I go about doing what you're suggesting.
Select File | Manage | Database | Fields
Select your table and add a number field for the record's exchange rate.
Double click this new field to bring up its field options.
In the dialog that pops up is a tab labeled Auto-Enter
You'll find you can specify a lookup, (requires a relationship linking two table occurrences)
or a calculation.
You may want to read up on these features in the help file.
Thank you kindly