When I draw a latitude LOP, I line up the parallel ruler on a lattitue line on the chart to ensure that the ruler is exactly horizontal. Given that longitude lines are NOT parallel, can I do the same with longitude to ensure the ruler is directly vertical? If not, what is the best method?
There are several excellent videos included in the Plotting module that will take you step by step through the proper methods of using your parallel rules.
In fact every element of chart plotting includes at least two video solutions which the student can watch as many times as they choose. The videos were intended to take a complete novice and give them the proper guidance and instruction to become master navigator.
Longitudinal lines on a Mercator chart are indeed Parallel. The longitude scale cannot be used for measuring distance since the Longitude scale would only be accurate at the equator for measurment purposes.