Question

To calculate the contour interval

subtract the two index contour lines, take the results and divide by the number of spaces between them
add the two index contour lines and divide by the map legend distance
subtract the two index contour lines and multiply by the number of lines between
subtract the two index contour lines, take the results and divide by the number of lines between them
add the two index contour lines and multiply by the number of lines between
add the two index contour lines and multiply by the map legend distance