It would be good if they explained it a little better.
Hmmm. I wouldn't wait until there is a 20 stitch repeat to redo the cable pattern. There are two separate sections of the cable, so you'd have to work the 4-stitch cable on the outside edges first. for the first 10 rows, and the the round cable at the center section of the chart. Know what I mean?
I think what I'd do is either draw and expanded chart, or if you have a photocopier available, make some copies. Then each time you are on the right side after an increase, work it as if the chart were growing outward. If you don't have enough to cross the cable, just knit it. When you have enough to cross, then do that.
Does that make any sense at all?