I get that they approximately traveled 1,400.15 miles. When clicking the red dots you get the distance from St. Louis to Ft. Clatsop, which is 5.125. I then multiplied 5.125 with the mileage number that was given in the scale for evey one inch, which is 273.2. So, when I multiplied the 5.125 with 273.2 it gave me 1,400.15.
