A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Fleming
Date: 2016 Feb 16, 11:37 -0800
Drawing the line at an angle of the mid latitude determines the ratio of a degree of longitude to a degree of latitude.
That same ratio applies to any number of minutes of longitude to the same number of minutes of latitude at that mid latitude.
Therefore draw the line corresponding to the mid latitude, then label the latitude and longitude ranges to have equal differences and everything will work out correctly.