The conversion rate in converting the volume of an order in quote currency to the volume in USD is calculated using spot prices on Kraken. Spot is taken as the mid-point of bid and ask. This calculation is for the purpose of calculating your 30 day trade volume in USD.
To get the spot rate for something like EUR/USD, which we don't have a market for, we use the spot for XBT/EUR and XBT/USD to get it. In a few cases we might not be able to get the implied spot rate that way and for those cases we pull the rate from a third party site.