Hi @Jon_G
Thanks for coming along! 
The foreign currency adjuster is actually a community developed tool, made by @anthonywilliams.
In general, currency fluctuations are calculated on invoices when they’re raised and when payments are allocated. With bank accounts, it’s a bit more manual (which is where @anthonywilliams’ tool comes in).
As long as the bank accounts are re-valued on a regular basis (e.g. month end), this should be sufficient, especially if you don’t have many transactions.
Hope that helps, but if you have any more questions, just let us know