Rates and currencies are taken from European Central Bank (ECB), meaning all currencies only have a conversion rate to EUR.
If someone need a currency conversion from USD to GBP, then it will be calculated USD-> EUR->GBP. A script will recalculate the conversion rates on a daily basis.A conversion will get a payout and a conversion rate on the given day of the conversion. A change in the conversion rate at a later time will not change this.
If there is a need, we can integrate another exchange to have better calculations for another base currency (e.g. USD).