which type of scale should the mapmaker use?

Question

hes drawing a map for use in 3 different countries that do not share a common unit of measure.

Answer 1

There are several standards on how to produce maps. The full answer would go far beyond the scope of this tool. The short answer is that the two main system of measure, the international or SI and the imperial should be used. The first is based on decimal scales for distance (meter), temperature (centigrade) mass (kilo). The latter is based on unit of measure initially introduced by the British and common in USA for distance (inch to miles), temperature (Fahrenheit) and mass (pounds). Please not that maritime maps are somewhat more complex with archaic unit of measure like fatom and so on.
Finally there are set standard in cartography on how to add legends and conversion maps so that if you are familiar with one system of measure, you can read the map and convert anything to the other system of measure that you are less familiar with, should the need arise.

Answer 2

You can't use miles in Europe and other metric-based countries. Perhaps you could do a combo like many US maps are designed.

Answer 3

1 inch to 1 mile