Place the ruler directly on the position in which you're interested. Position the ruler horizontally. A trick to achieve this is to align the lines on the ruler and the vertical longitude lines.
Mark a dot on the latitude scale to remember your position. If it falls directly on a latitude line, then you have found your answer. If not, use the minutes and seconds scale (60 minutes equal to a degree and 60 seconds equal to a minute) to determine the exact latitude value of your position.
Divide the space between the two latitude lines into 10 sections, using your ruler. If your position falls inside a section, divide it into 10 subsections and express latitude as a decimal (+65.35 for example). It is an acceptable method to express the exact latitude value without using minutes and seconds.
Measure the distance between two minutes of a degree using your ruler. If degrees on the scale don't feature subdivisions already, measure the distance between two latitude lines and do the division by 60.
Use the measurement to express any distance on the map in nautical miles. For example, if a minute of latitude is 3 inches and the distance between the lake and the town center is 27 inches, then the real distance is 27/3 = 9 nautical miles.
Convert nautical miles into miles, as this is the unit used to measure distances on land. A nautical mile equals 1.5 miles. Therefore, on the previous example, the distance between the lake and the town center is 9---1.5 = 13.5 miles.