Coca Cola is ubiquitous. Its available in every country, in every city and rural town. You can buy it in shops, or from street vending machines. Its everywhere.
As a low value item, I use a can of coke as a cost benchmark. If I know the cost of a can of coke, I can make estimates on several factors, such as practical currency to carry, cost of living, etc.
My strategy is simple. When I exchange my home currency into the foreign currency, I simply ask one question: Outside the airport, how much does a can of coke cost?
For example, in Japan, a can of coke costs ¥130. The smallest denomination is ¥1 and the highest denomination is ¥10000. So you can work out what denominations to carry based on the cost of one can of coke – I would recommend, in this case, to carry more ¥1000 notes (the smallest note available) and only a few higher denominations (such as ¥5000 or ¥10000).
Another example, in Australia, a can of coke costs AU$2.50. The smallest denomination is $0.05 and the highest denomination is $100. In this case, I would recommend to carry more $5-$10 notes.
A can of coke should be low value in comparison to food. So if the price of coke is low, then the price of food should closely correlate. This is so true in all of the countries I have visited.
For example, in France, a can of coke costs €3. This is relatively high in comparison to the rest of the world. Therefore, the cost of food will be relatively high. True.
Another example, in Spain, a can of coke costs €1. This is relatively low in comparison to the rest of Europe. And true enough, the cost of food is relatively low.