Ideal line length

Hypothesis: ideal line length is in the range where you can peripherally glance the end of the line while fixating on its start, because (further hypothesis) this allows your focus to travel toward a fixed point rather than carefully trying to stay in the narrow track of the line.

(From this would follow that large leading only helps by mitigating the difficulty of tracking an over-long line, but leaves the underlying problem of a missing destination fix point unaddressed.)