just my two cents on this:
In my first travel with TravelSpend, I thought about the same. I have made several withdrawals from ATM around Japan and wanted to add these amounts of cash in the app. But no matter how I thought about it, the method wasn’t right. So I decided not to add these withdrawals, at all. Why? Because I add all the expenses into TravelSpend and can even say which are made via credit card and which are made using cash. So I make a credit card withdrawal of e.g. 120 thousand Yen, then I spend some money and add these expenses into TravelSpend. So in the end you would have something like this:
You’ve withdrawn 120k Yen, your cash expenses in the app sum up to 80k Yen, so there have to be 40k Yen in your wallet. And you keep the receit from the ATM, so you can a) make this calculation and b) check at home after the travel that the bookings on your credit card account are correct. Then you throw them away and the details of your spendings are in the app.
That is how I usually do it.