Take a piece of string. Any string will do - sewing thread, shoelaces, guitar strings - it doesn't matter as long as it's not elastic.
Measure distance with piece of string, then hold string against the left or right side of the chart (note: not the top or bottom). One degree of latitude = 60 miles. One minute of latitude = 1 nm (or near enough for practical purposes). Bonus points for marking the piece of string in 1 nm intervals.
Unless you've got a single leg that stretches from a bottom corner of the chart to the opposite top corner, the scale on the left/right side will always be long enough.
Technically speaking this method is even more accurate than using a 1:500.000 marked ruler, since your chart isn't a uniform 1:500.000 throughout. However in actual practice the difference is so small as to be negligable. (The winds aloft forecast is less accurate, for instance.)