Show aggregate for custom exchange rates

I’m struggling to keep the correct exchange rate in my bookings and therefore my stats don’t add up. Before my current trip started I charged my debit card with a budget in foreign currency (TransferWise) which means that for the majority of bookings I have a single custom exchange rate. However, I find incorrect, dynamic exchange rates for almost every day. That means I have to go through each and every booking to find the offenders. With an aggregate similar to "cash"and “credit card” I could resolve my errors a lot faster.