that's when you use a tool called dividers. i'm sure you've seen them in many naval movies when a course is being plotted. they look like a compass (the sort you use to draw circles) with two points.
1) draw a straight line between the points you want to measure.
2) set the dividers at a convenient distance.
3) start marking off the distance by flopping the dividers along the course line.
4) when you come to the end, reset the dividers to the remaining distance and read that last bit off the scale.
