On row 13 of the cable pattern, work the first decrease. Then you work 19 more rows total, so it would be row 14, and then rows 7-14 twice more, and rows 7 and 8 -- you've completed all 19 rows. Then work the next decrease if this is where it falls.
I find it helpful to make a chart to keep track. In a column write the rows 7-14 over and over, and then mark which rows need to be decreased.