I am using Irish Central Bank Rates to calculate value in Euro for foreign currencies. For example, if you buy something in UK and you have invoice/receipt in Sterling (£), you can find the date of purchase in table below and use the rate to calculate Euro value for accounts purposes.
For example, if you pay by credit card the better way is to use the rate on your Credit card statement. If you pay by cash and you exchanged Eur to Sterling, you can use rate the Exchange office charged you. Otherwise, if you cannot identify the exchange rate, you can use table below.
FX rate = foreign exchange rate
Source:
Central Bank of Ireland’s website – Foreign exchange rates
Foreign exchange rates from Central Bank of Ireland from 2016 to present (MS Excel document)