Would be great if there was an extra tab for when you were travelling, so you could convert from euros to pounds on the exchange rate you are going to use.
I know I often stand in a shop abroad trying to work out how much something will cost me. I know you can use the MasterCard website, but I find it very badly done.
I think Monzo charge you the MasterCard exchange rate on the day the transaction settles, rather than the day it’s first created (I’m sure someone here can explain the payment cycle better than I can). So Monzo don’t actually know what the rate will be, since it could take a day or two for the transaction to settle.
Monzo does use MasterCard exchange rate, and while abroad it will inform you when you spend on the card what it converts too (eg 10 euro - £9.55 or something like that), then after a day or so it will come out correct but it’s usually pretty close if not out by a few pence.
So an exchange rate converter in-app before purchase would be useful as it would give you a pretty good idea of how much it will cost to within a few pence 
Agreed. It would be a useful tool as long as it’s made clear that the rate is indicative and subject to change when the transaction settles.