Yeah, it would be nice if the game gave you accurate "as the crow" flies distances on the map. You can calculate the march distance from point A to B with a unit that moves the same speed over the whole path. Just take the march time in hours and multiply it by the km/h of that unit. This will give distance in km. If you have a plane that will reach a point of interest you can do the same with a plane by sending it on patrol from point A to B. I have also used the plane attack range circle at a given map zoom level to calculate km/inch for example, in the vertical and horizontal directions. Then to estimate an "as the crow flies" distance you can use a ruler on the monitor and derive the distance in km.

maps without scales are peculiar to CoW and real estate sales.
a simple horizontal and vertical coordinate in the LLH corner would suffice.
grid lines have been added to some oceans, but no labels!

I wouldn't trust those grid lines on the ocean too much either; they're more of visual effect. I wouldn't be surprised at all if they yielded different results when you're closer/further from the poles, or in the horizontal/vertical direction.