[peterwhitesell]

Even when typography isn't bad I've always had a hard time tracking back to the next line both with text on paper and on screens. My most common use is to highlight one line at a time, although I sometimes highlight to save my place (arbitrary length selection) and sometimes to emphasize a concept out of a larger mess of text (context based selection length).

Usually I highlight from near the middle of one line to near the middle of the next line. If near the end of reading line N I highlight from the middle of line N+1 to the middle of line N+2, I can use the one highlight three times to help me track back: first I track back to the first line above the highlight (N+1), second I track back to the highlighted line (N+2), third I track back to the first line under the highlight (N+3).

Similarly highlighting multiple lines at a time it's easy to subconsciously remember which line of the selection to read next (first, second, third, ...) without having to visually track back, as long as the selection isn't very long (much like how it's harder to keep track of where you are in a longer paragraph).