https://www.xe.com/currencycharts/?from=USD&to=GBP
It is a moving goalpost, the above website is good to check as a proxy for exchange rate pairs.
Your FX Impact is the difference between the current market exchange rate, and the average of all the exchange rates each time you purchased the stock. It doesn’t matter if you made one purchase or 20, what matters is the exchange rate at the time of each purchase.