Backbearings with a compass

Here's an exercise I gave to my North Shore Paddler's class on taking backbearings with a compass.   

We prepared a chart with magnetic variation lines for Boston Harbor drawn in.   The exercise says that you take a bearing on the Boston Harbor Lighthouse at 30 degrees magnetic.   Plot the back-bearing just using your compass.   

The first step is to ignore the compass needle.  You rotate the dial by 30 degrees.  

Step 1: prepared map, rotate compass dial by angle of bearing. 

Now, for step 2, flip compass around 180 degrees and align the orientation lines on the dial to line up with the variation lines.   Move the edge of the baseplate to the object you took a sighting on, being careful to keep the orientation lines parallel to the variation lines.   A line extended from the sighted object (lighthouse in this case) gives you your line os position.   Couple this with a range, and you have your position!