I forgot about the.... continue without shaping until piece measure 20cm part. This just means that after you finish those decrease rows, you will continue on in stockinette stitch (knit the right side rows, purl the wrong side rows) with NO more decreases, until the piece measure 20cm from beginning of armhole shaping, which is 7.87" (thanks to an on-line calculator!!
). You will end with a WS row, meaning you will end having just completed a purl row.