Beauty in letter construction, as in life, depends upon relationships between the parts of a whole. Visual relationships depend upon patterns of consistency, either in contrast or harmony. Johnston chose to aim in the first place for harmonious rather than contrasting patterns. Balance is a clearer target to aim for than contrast for balance is theoretically achieved around one point, whereas contrast is a sliding scale.
Johnston eventually emphasised three technical conditions for achieving balanced and harmonious calligraphy: letter weight should be consistent, as should the angle the pen is held at and so should the repertoire of pen stroke shapes underlying any given alphabet. These factors are usually abbreviated to consistent weight, angle and form.